In Animation::UpdateFInishedState(), per the spec step 1 in [1], we are still using animation's current time, instead of the unconstrained current time. This may cases the issue:

Note: This is required to accommodate timelines that may change direction. Without this definition, a once-finished animation would remain finished even when its timeline progresses in the opposite direction.

We have to update this function and probably check the test cases. We may have to add some tests if we don't have them in wpt.

[1] https://drafts.csswg.org/web-animations-1/#updating-the-finished-state

Yes, I believe that was added for scroll timelines so we will need to implement that now.

Will do this in Bug 1775327 together with other patches