Tristana immune to cc during leap.

I was playing a ranked game today and noticed that I was unable to cancel Tristana's jump. Not only this but the cc wouldn't last after the jump either. I was playing Thresh and there were at least 3 times when Flay failed to stop it when I felt it should have, although it is possible these missed. The biggest issue was when I landed Death Sentence as she started her jump, but she was able to complete her jump anyway, and was able to flash and move normally after the jump finished even though she was still hooked, as if she has been given by a Morgana shield. This turned a free kill and tower into a lost team fight. Sad face. Anyway I was wondering if this is a change that was intended but hadn't been announced or a bug or just a freak incident in that one game. Thank you.