Mobs stop moving for guests in server when host player is certain distances away

When the host of a server travels certain distance from mobs they stop work.

I expect this is normal behaviour for single player hosted servers, but seems out of place when hosting a game for your family. I move away to do one job in the world and mobs stop working for every one else.

I'm more then happy to post maybe a video of this happening. Or provide stack traces if necessary .
Regards