@Nine IIRC we also don't know that well e.g. why unrolling scotch tape emits light, why steam bubbles emit light when imploding, how exactly does the mechanism behind thermoluminescent dosimetry works (we have some models that very obviously can't be correct for some setups). I expect that there's lots of similar things in solid state physics.

In general my impression was that these things are present the most in cases where the whole setup has a complicated description at the level that's relevant to any reasonable explanation. I expect that you can find a similar set of weird intermediate-level very hard things in e.g. biology, too. Also in math: there's lots of number theory conjectures (e.g. Collatz conjecture) that seem very simple at a first glance.