I had a dig around animation changes in 65 but didn't find anything suspicious.
Bug 1488122 is about the most significant change I can see there but nothing about it seems suspicious. It could cause extra restyles in some cases (but only when there is an implicit from/to keyframe--all other cases are not enabled on release builds) which could cause some elements to live for an extra frame but the crash here seems more likely to occur if something was released earlier, not later.
As for the AnimationEvent class itself, it doesn't add anything interesting to
WidgetEvent and hasn't been touched for quite a while. Perhaps more importantly, as far as I can tell we haven't changed anything related to its dispatching in that time frame either.
(For my own notes, the stack in comment 0 suggests the AnimationEvent dtor is calling the InternalFormEvent dtor but if you following the source link it appears it is actually the WidgetEvent dtor that is called.)
Looking at the other crashes for ~AnimationEvent over the last 9 months or so it seems they're all on Windows:
There doesn't appear to be any change in their volume over time (unlike crashes with the ~InternalFormEvent signature) or by release. In the ~InternalFormEvent signature is just a variant on ~AnimationEvent, then 65 might not be the right time frame to consider.