I have a train station after my oil processing, which is also intended to load sulfur to cargo wagons. Currently every time the rail tanker stops at the station, it loads 100 sulfur (stack size might be modified by other mods) into the only storage slot of the wagon; it also gets transported to the destination, but disappears there. (At the destination there is only one pump per wagon, moving the fluids into tanks, no grabbers.)

That's not a bug, but a necessary drawback to make Railtanker work with the full/empty conditions in 0.13.
If you don't need that functionality for the tankers you can use https://mods.factorio.com/mods/GotLag/Tank%20Wagon