I am not sure why, but during playing with Scout on a heavily modded Fabric client the satchels and pouches just tend to freak out and revert in time their inventories, deleting or adding stacks of items according to a state they randomly memorized. Sometimes things change just visually and you can click on an empty slot to get the item that was supposed to be stored there pretty recently. Sometimes I open Backpack from Inmis mod and the fake inventory in the pouches becomes real, duplicating items, and deleting some really valuable ones. I can't rely on the pouches to store my valuable information at all!

Here's my modlist: (FABRIC 1.20.1) modlist.txt

I can also confirm it can happen on 1.19's version, so this is a longstanding issue. Just lost a bunch of items.

Can confirm, I'm also having this issue on fabric 1.20.1 in a server environment, data loss seems to happen especially commonly upon leaving/rejoining the server, but I've also seen item desyncs occur when changing dimensions, as well as when switching from survival to creative and vice versa.