I think this is on your end But for some reason, I keep getting mobs that don't die and fall over (this is about 1 in 25-100. It is possible its another one of my mods that is causing it in the 1.15.2 version but and easy way to test is to be in creative in a planes biome at night and type /kill @e[type=!player] (this is the kill all entities command besides the player) If you can replicate it then that is the issue I have found.

Closing as I haven't heard back. Let me know if you do end up with the modlist and can repeat this bug, I have no idea what's causing is still.

Can I get a mod list?

will grab one.