Issue paired with Deadlock's belt boxing ( https://mods.factorio.com/mod/deadlock-beltboxes-loaders ) The stacking belt boxes are affected by the additional productivity.

Hmm.. i think maybe the error is related the option "Multiply Stack Size" in mod settings. See if you return to normal if you de-select this option, please. Thank you

Mod version 0.0.18 for game 1.1, and productivity still affecting DeadLock's Beltboxes.

DeadLock's mod registers the packing machine in the file "prototypes/create_beltbox.lua", and the entity type as "furnace". And as your mod changes the productivity of all furnaces... i can't blame anyone...

For me this code i made fixed: https://pastebin.com/D84MwA4f , also, for my security:
This code was made in the terms of the license of public domain "The Unlicense".
I am not responsible for miss use of this modified code, lost saves or any other event.
Use this code made in 15 minutes and poorly tested at your own risk.

I'll try tuagolegals code. Its unplayable otherwise because its way too easy then