Mitch K

This probably is not the same issue, and I don't think the same unloading rules apply to the players because the player is not saved in chunks and can't be unloaded.

This is not the same issue. Entites (including item frames and armor stands) are occasionally rendered (or rather, not rendered) completely invisible, not semi-invisible.

I've noticed the same thing. I've even gone through the entire lunar cycle, and I only got one slime to spawn total. All caves below ground are lit up.

The mobs become visible again, but not permanently. The mobs will eventually become invisible again after leaving the area and returning

I tried doing F3 + A when the mobs became invisible, and the chunks did reload, but the mobs did not reappear, which suggests that this issue is not closely related to that issue.