Mod Version: 1.0.1
Forge: 2758 | Custom Pack, but not any major differences from mainstream 1.12.2 packs

When traveling to another dimension the backpack seemingly disappears. It is no longer rendered on the player body, is not accessible via the hotkey, and can not be removed using the hotkey.

Disconnecting and reconnecting fixes it and then seems to be able to travel back to the overworld and it will work fine. But as soon as going to a dimension again it breaks.

I've tested with the End, the Nether, and various RFTools dimensions. Have tested using the vanilla portals for the End and the Nether as well as the Charged Porter from RFTools.

Yeah I've noticed that one too. I just need to sync it back to the player.