Sion Ult Bug

So I've been playing Sion a bit because I recently got pounded by one in lane, so I wanted to try him. I was using the normal Sion no skin and it seems to me every time I use my ultimate point blank range I have to press R again to stop the animation channel. The damage goes through and so does the cc but the channel continues like I'm going full range and wont let me cast abilities to cancel the animation I have to press R again to end it.messed me up getting a few kills. Not too sure what to do but hope I explain it enough. TY.