Think I didn't fully comprehend some of your initial comment but just now reread it. The thing I was mentioning is older I think started around 9 months ago. It doesn't happen on every ult but happens enough to make you wonder what the heck is up. A decent enough number of ults in the past have been wasted because your guy is standing still and you cannot make him do otherwise just long enough that your not going to get off a fear or reposition or anything else. Think i've gotten use to it by now but it sucks.
As for what you are saying, seems you are saying you can no longer cancel out the ult by flashing during the channel of the ult? I'd be happy if that were true but would be afraid to try it due to a unintentional timing issue I somehow got myself into for like a straight season where I was canceling the ults all the time where up to like mid season 5 i've never done that. Then mid S5 did it once and got on a long run of doing it all the time and couldn't stop it. It caused me to think that they'd made a change where prior possibly you could buffer flash during channel and even read a comment where someone suggested same. Though other comments from prehistorical lol times suggest it has always been as it is now and I just somehow infected myself with a case of the fid ult flash cancel blues.
I don't think they consider using flash during fiddle's ult channel to cancel it, to be a bug so i'm hesitant to believe they 'fixed it' though i'd be happy if they locked you into the channel and denied you the ability to flash cancel out the ult during the channel, and would be happier if the end result is that you could buffer a flash during the channel that took place when ult completes. I'm not noticing anything different lately then I've seen before so i'm not following you on the flash thing. Wouldn't be first time i've adjusted to some small thing that went unnoticed by me until someone pointed it out way later. But i'm not experiencing a change that i'm noticing.