Ronzik

I'm not certain of this, but I think this has been in the game since 1.15, and I think it is purposeful.

There are many possibilities here. The one thing I will say is that you managed to get two screenshots at different angles of what I'm assuming is the same endermen in flowing water. Since it was in flowing water it would have been moved by the water in the time it took to take both of the screenshots. It is possible that you are playing on a server that has a lot of lag or you may have high latency connecting to the server which resulted in this. I'm certain there are other possibilities, but I don't think I know the correct terminology so I won't try to explain them.

You are playing on a server which means the server is the one loading the chunks. There is no way to improve this unless the server changes hosts or is upgraded. Some servers have set render distances and so they will not render chunks/entities further than that distance, even if you have your settings higher. I'm pretty certain this is how it works, maybe I'm wrong.