As the title says, mobs can fall through floors in mob heavy chunks (a guess of why it happens), example, mob farms, seen in the video attached. Has happened to me on essential servers and in single player worlds. I do not have any server effecting mods, only client side. Has happened in 1.19.3 as-well.
THIS IS NOT A VISUAL BUG, THE MOB CAN BE INTERACTED WITH.
The video metioned
The world file
Mod list if needed
IT MAY TAKE AWHILE
This bug happens semi rarely, and sometimes i can spot mobs falling out of the corner of my eye. Try spawning mobs on the platform seen in the video and wait for it to happen.