The interaction between annie and tibbers stun is currently bugged as of 6.19. The interaction at play is supposed to be gain four stacks of annies passive ult and the result is supposed to be a stunned aoe area. currently this works maybe 15% of the time. most times it just wifs the ult and make you look like you missed. other times itll stun but for a half second.
Just to clarify - you are claiming tibbers is not properly stunning when used with the passive?
Some things to note - there is a slight cast time (.33 seconds if I recall correctly) so enemies have a slight chance to move out of the AOE. Also, Annie's stun time increases based on her level, so early level stuns are shorter than late game stuns (might account for the second issue).
Additionally, if you have lag spikes or frame loss, it can appear as if the timing of the spell is off.
I haven't had problems with annie this patch, so it might be something like that.
