In Spigot 1.17, the mob cap appears to be nonexistent. At first, it seems as though the server is running normally, but after around an hour or two, there is lag, which some players may not even notice. Going into spectator mode and looking underground reveals hundreds of mobs despite the hostile mob cap being set to 60. This is a huge issue as the only way to get rid of them is by using /kill which can potentially kill named hostile mobs. 

If any other info is needed, let me know and I will provide it. I attached screenshots from in-game of the issue.