I saw this on singleplayer also. It seems to be a difference between minetest 5.1 and 5.2. Running the same code on each, I saw that mobs flashed in 5.2, but not in 5.1
It started happening after I updated the mobs_redo api. I think there is a new feature in the mod that causes it. Not sure if the version I have is glitched and I will update soon and retest and if it still happens we'll dig into the mobs.