When having a bottle or canteen in the "quick bar", if you select it and then use it(drink from it), you take a swig from it and then it gets de-selected.
I would expect it to stay selected. I mean, I don't expect the rifle to be automatically holstered after I fire a shot, right?
Typical scenario is that I find a well, drink what is left in the canteen, and refill the canteen from the well.