Sounds like it could be the Net Cull Distance Squared value on the characters themselves. If your distance to the character exceeds the net cull distance then that character is removed temporarily until the distance is reduced again.
The default value is very large to begin with but try adjusting it to see if that helps. Just know that increasing it comes with potentially more network cost and wasted bandwidth especially if you can’t even see the actor in the first place (behind a hill) but it still is replicating data (within net cull distance).