"wait! I can explain!"

No waiting or explaining ensues

Often, but not always, yes. If the misunderstanding is reasonable and they had ample opportunity for an explanation but didn't, then yes, it's an idiot plot.

If they legitimately didn't have an opportunity to explain, or the explanation, while true, was completely unbelievable, that's not an idiot plot IMO.

Only if 3rd parties who witnessed the context of such misunderstanding re-enforce the stupid. Otherwise that's just your average contrived romance drama.

I feel like there are so many plots that stem from simple miscommunications or a lack of communication between two charatcers which would easily resolve the whole situation.

That's similar to the one where the couple breaks up over a misunderstanding and all it takes is one of them explaining in like 10s. But instead someone storms off while the other pleas, and deep down you're like, "That motherfucker so wanted it to end anyway."

But then we deal with post-break up scenes and you realise they're just idiots that still haven't even thought to clear up the misunderstanding via a single text, call, or friend. It could all be resolved, laughed about, and fucking again like that afternoon.

I think idiot plot is supposed to be so stupid that it's essentially not possible to happen irl

The example you outlined could happen in real life if one person completely blocks the other or something

Would be an idiot plot if everyone involved is fully able to communicate to the other parties, everyone has plenty of information, and they decide to go through with whatever stupid shit they think they need to do, that would be completely unavoidable and anyone with half a brain would understand that they don't need to do what they think they need to do.

Like a movie where there's a killer on the loose and nobody thinks of even trying to call the police. No "oh there's no signal here", just straight up nobody even tries.