*spoiler warning*
i died while i was bringing back the Firetank fuel things, to a hog. When i reloaded, both of the fuel tanks were at -1.0 fuel. Trying to use them set the gas can to -1.0 aswell, and it registered as empty.
I imagine it's some kind of bug relating to the new Firetank fuel system, this also happens when you spawn one of the fuel tanks in sandbox.