This has happened twice: all of the paintings in my world vanish, though they are still present in the selection menu.

The first time was after copy-pasting a selection with worldedit/mcedit, I can avoid this.
But the second time occurred after installing a resource pack: https://www.planetminecraft.com/texture-pack/custom-japanese-weapons/

I find it very strange that a resource pack caused this. It is unclear why this specific resource pack caused this.

No, I didn't receive logs

I'm not sure if I have the log, but do you remember what caused the paintings to disappear in this ticket?
#24

It looks similar

Without the log between the moments the paintings were still there, and then vanished, I cannot help here