"Invulnerable:1b" tag breaks paintings that are larger than 1x1 block (though I've seen a few cases that some 2x1 paintings did not break). They don't always pop and drop as entities. Most of the times, actually, they just dissapear.

Recreate the issue : place down paintings of various sizes and run :

/execute as @e[type=minecraft:painting] run data merge entity @s {Invulnerable:1b}