Once upon a time my stepfather had a friend which I liked very much. At some point I would ask him if we could visit this friend again, and he said:
“I don’t think so, he never calls.”
And I thought: Oh my, if your friend is thinking the same way you do - neither willing to call first - we’ll never see him again. This is exactly what happened and I thought no more of it.

Later on I had to find out this was not a singular event. It’s a thing, in everyday communication, and far more complicate then I had anticipated at the time. Though the situation does not come up for just everyone. In a sense, if you’d allow me the generalization, there are two kinds of people in matters of friendship. One type takes a friend as a friend once the person in question is a friend and recognizes that person as a friend until some injustice or damage brought the friendship down. The friendship is like a structure, build to last unless destroyed.
To the other type the friendship is like a plant, it has to grow and needs a lot of routinized care. Should the friendship be neglected for whatever reasons, it decays over time until it is no more, and no matter what has been between them before has to be formed anew from scratch.
Both types are totally valid and have their pros and cons, no question. And in people they are not just this or that one, but a mix of the two with a tendency to one side at best. Mind that the tendency might be different depending on context, so an online relationship might be more like a plant while a relationship on a con that takes place ones a year might be unaffected by the whole year of neglect.

So where’s that Anomaly? Well, as my introduction displayed there are times when both parts of a friendship want it to last, AND both are waiting for the other to call first. The friendship decays while both stand ready with water in hand for the plant. It’s like starving with full plates before you!

If this was a handbook for friendship, I’d just say “Never wait for the other to say hi first. If you miss your friend, say hi yourself.”

But it’s a journal, and the matter is far bigger than what I’ve said so far, or will have said in total.
I’ve lost a row of friends myself to this, but mostly because I am the structure type through and through and never understood the plant types for many years. There was a person who I really liked, and he liked me very much in return. I’m a talkative type, he’s a quiet type. Compared to my talkativity most people are the quiet type. Many, like that one, find comfort in it. I speak my thoughts openly, and they can listen to what I ponder or feel excited about. This one friendship was a significant example of where such situation leads:
I was talking all of the time, the other was listening all of the time. I chose every topic and lead the ‘conversation’ in a monologue. Never before had I thought about it like then, when at some point the whole thing felt incredibly empty. Nothing was returned to me, and though I can take that for a long time, it didn’t go without leaving impressions. If there is no reaction ever, I start to believe my topics are boring, or I am boring, or the whole thing is not really of interest to the other person whatsoever. In this significant case, I had built half the house alone. And then, because I was done, I waited. Rather, there was no topic or thought I hadn’t explored to the fullest left! My head is so full with thoughts I’m still dependent on writing diaries to get a hold of all of them. Nonetheless, after four months and a half, for this person my head was empty. I told him so, asked him to pick topics. None came, and at some point I was told our friendship is over – because “You never call”.

This happens more often than you’d think, I noticed. To me at least. People give some thoughts in return, and the more they do, the more our friendship is growing – for me, at least. I should highlight that friendship for me is entirely build on conversation. We could play games together for several years, or develop our own projects in any craft, if after all that time I know nothing about YOU, I won’t feel a shred for you. Others will love you like a sibling. So it is very dependent on person and circumstance what counts as taking care of a friendship and what doesn’t.
In my world, where it’s conversations, I’ve had it often that I chat with people. Since they do but listen, there is sometimes no answer at all or any variation of “Hm”, “hmm”, “hrmm”, “mhm” and so on (I can tell some people apart by just these letters). These letters are still better than nothing. Because if I build up a topic and say I want to talk about that, I wait for yes or no in the least. That can also block my part of watering the friendship. There’s topic A, and B, and F and P which have not been solved yet, still waiting for a reaction. So I look at a chat-log and just see those topics I still want to talk about, waiting for “ok, go ahead” or “later we can use voice-chat” or “how bout we discuss that over a cup of coffee, it’s just half an hour to my place?”.

What happens more likely is that:
“I noticed you didn’t say much recently. I respect you need your distance.”
Or just
“You never call.”

Of cause I fume at that! But point is: Same Anomaly, other color. For one person friendship is depending on this or that, for others it’s different. In my personal case, I wait for something, and the other person doesn’t even realize there is something more or less important I am waiting for. For them, their part of taking care of the friendship was done with listening. So yup, to some people I’m a tricky experience. They think I just need some ear to stuff with words, so they lean back convinced they are there for me, while in truth they neglect me with their silence and remain an empty sheet of paper.

Thankfully it is indeed an Anomaly, meaning it’s not the normal circumstance. It could be stretched on and on in many directions and details, but the point is made and I can only advice to treat your friends with an open eye (though most of the time you can just follow your instincts). As a rule of thumb I can tell any form of this Anomaly is provoked by silence. Thus it remains that if you miss your friends you should call them, no matter how you feel about “whos turn” it is.
For me, as the building type, a friendship is not broken by silence. There are friends from my childhood I haven’t spoken to in over 15 years, because we lost us, yet I know I’d be as welcome at his door as he is at mine should we ever find us. “Take whatever you can get”, we’d say, our old parole, and the other would respond with the correct answer and a hearty laugh.
However, I lost many people because they were different and I fail to understand the plant type in full to this day. The friendship would last to me, but give me one sign that silence is a form of neglect to you and I’ll go wild with anxiousness – because I would not know how to keep talking once my head is empty. I’d be doomed to kill the friendship for you at some point, no matter what, no matter how much both of us don’t want it. A grave dilemma, where some would say “if it’s not meant to be”… Well, screw that, if it can be predicted, it can be prevented.

~Cheers!