i dunno either. but i have used swiftcast then have had a spell interrupted. and the stack of swiftcast was indeed consumed for that spell. have had this happen to me several times.

i think something is wrong with the game or UI. i have had even more numerous occasions where my spells were interrupted for ZERO reason. just be standing there with the target tank in front of me. tank not moving out of LoS. i am not getting hit. i don't move my character. just sometimes spells stop casting for no apparent reason.

i think the game is just broken.