Minecraft bugs on the server

So with the latest update, I have found a few issues. I can't pin-point what is causing it. Wondering if someone can help...

So the server I own is just a basic one, holding a max of 20 players. I have the standard plugins most people use with a few extras. Nothing major or complex.
I noticed that since I updated everything to a 1.11 server, mobs aren't always showing. Some players aren't either. What can be causing this?

This sounds like a database issue. Check each part of the database to ensure they are properly being connected with the player information.

Okay... Is this one solved?

Thanks for asking, but no. Does anyone have any opinions? This bug is really annoying me DAILY!