I've been working on a custom modpack in recent weeks, which includes Alex's Mobs. I've had a lot of mysterious issues, but two in particular I finally found a source, which seems to be this mod's Skelewags mob. They seem to not only be spawning in in very excessive numbers causing an impact to server CPU, but they cause issues with the many drowned that spawn with them. Console consistently gives this warning: "UUID of added entity already exists: Drowned".

That's the shortened version, naturally, but that's what gets spammed over and over. Didn't link it to Alex's Mobs until today. Turned the server to peaceful, Skelewags (and riders) vanished, as did the console spam. Turned it back to Normal, waited for Skelewags to spawn again, and sure enough it started back up. Repeated but used a Mega Torch to prevent their spawns while in Normal, and not a single further one of those errors.

Similarly there was also an error with their Drowned riders when used in an Industrial Foregoing "mob duplicator", where Skelewags would still spawn with riders that would then get stuck in mid air, rapidly overlapping with hitboxes, and requiring a full server restart to get rid of. My bet would be that there's some sort of connection, hence why I'm mentioning it.