Yeah, as far as I can tell there is a skill q priority that comes into play when activating skills (defensive ones like blocks and heals seem to have a higher ones, also some CCs), so some skills will cancel others instead of q'ing, and/or don't get canceled by others with higher priority. Med kit f1 does not have a higher priority assigned afaik. So activating certain other skills before the cast has finished will cancel it. These priorities also seem to apply to general server skill lag - skills of lower priority are more likely to not activate or do so with delay.
