The backpacks seem to be having trouble syncing because I got a backpack in creative put stuff in it grabbed another of the same color and it worked fine. But then I tried adding another item in one of the backpack and it doesn't sync anymore. It even worst in survival because it doesn't sync at all. This is in version 1.16.4-5.3.0 of the mod.

Here is an example in creative

Here is an example in survival

This is correct, Bags currently no longer sync, This will be returning soon as an upgrade to the bag