When enter the end dimension, if you put a water in a block, this remains static as a solid block and not flowing.

After doing some testing today it looks loke setting pauseEventServer to false in hordes-common.toml, I'm working on a fix to make that option still work without causing the issue.

Which version?

This mod doesn't touch anything that would even affect it, so I'm pretty sure it's not caused by the hordes.

This occur only when I'm using the hordes mod in a server and play in multiplayer.
The version is Forge 47.2.20 Minecraft 1.20.1
The hordes mod 1.40

I tested it only with the Atlas lib mod and The hordes mod and the issue was replicated

See the image bellow

fixed as of 1.20.1-1.5.0