When my friend puts items in his backpack, they appear not to be there until he picks one of the items up with his mouse whilst his backpack is open. I.e., his backpack appears empty until he interacts with any of the items in the backpack, at which point they are all visible until he closes his backpack and re-opens it. This is really annoying for him.

We are playing over LAN. This does not affect me, the LAN server host.

Mod version: 1.0.6
Minecraft version: 1.12.2
Forge version: 14.23.5.2847

I have also seen this issue, but I am using a server host so even I get it.