Join any MP server that currently has players on. No monster mobs will spawn except from spawners, and mobs that spawn with an environmental building. E.G. drowned will spawn with sea towns.
 
No other overland mobs will spawn. Not at day, night, underground. This is not the invisible mob problem, these mobs are not spawning at all. Nether mobs spawn as normal.
 
Now go back to the same mp server when no other player is on. Mobs spawn as normal now. 
 
Passive animal mobs are not affected. Phantoms are not affected.
 
Tested on 3 servers.

Edit. Ok this is also affecting other things. 1 player on the mp server = Few problems. 2+, then no crops grow, no leaves decay. The main problem under discussion here though is the lack of mobs.