Building Gadgets 2 1.0.7
Sophisticated Backpack 3.23.8.1215
Sophisticated Core 1.2.38.936
Bigger Stacks 1.0.3 all

Prerequisites:
There are two Sophisticated Backpack backpacks in the inventory. The first backpack contains more than 64 of Item A, and the second backpack contains at least 1 of Item A.

Reproduction Steps:
Use Building Gadgets 2's "Exchanging Gadget" to select any non-Item A block, then use it on an Item A block.

Result:
Let the number of Item A in the first backpack be B, and the number of blocks exchanged by the gadget be C.
The second backpack will gain (B - 64) × C additional items.

This issue has been around for a long time, and I only recently figured out how to reproduce it.

I created issues on the issue pages of all three mods.

The authors of Bigger Stacks have solved it, the link is here: https://codeberg.org/PORTB/BiggerStacks/issues/59