The patch note says that you can only carry 1 stack of the explosive barrel. In reality, the ingame behavior is really inconsistent.
there's 2 different outcomes
1)drag and drop you can only pick up 1 barrel. (probably the intended method)
2) you press t to transfer while mousing over the barrel. you get the full stack of 10 in your inventory. shift+t works to give you half of the stack (5 or however many it is)
I assume 1 is the intended behaviour. Currently its bypassable by the most sensible way of doing things faster, by using keybinds.