Im using the neoforge 10.1.15 version of the mod for 1.21.1

Placing buckets in the in/out slots in a specific way causes buckets to duplicate

Placing an empty bucket into the top slot while a water bucket is in the bottom slot doesnt fill the empty bucket and allows you to remove the water bucket without the empty bucket filling. You can then place the water bucket into the top slot, causing the empty bucket to both fill and be grabbed by the mouse, duplicating it.

The same can be done holding a water bucket in the top slot, spitting out 2 buckets instead of 1

https://www.curseforge.com/minecraft/mc-mods/travelers-backpack
Fixed in 10.1.16