When a player opens the inventory, they cannot see the item stack weight.
But when a player loots any other lootable bag/crate/corpse, and keeps the crate open, they’ll be able to see item weight for stuff both in the inventory and the crate by simply selecting the corresponding item of interest.
EDIT: I just found out how to get the information blade on the right hand side back:
Open the inventory, go to a weapon, open its attachments, exit back into inventory. Voila! However, the item that you were editing/looking at attachments for, will show an incorrect stack weight but will return to normal once you deselect it and select it again.
Still, I believe there’s some bug extermination left to do