Uncovered with the updated ui framework forms:

Previously whenever an asset is edited, its last mod date would be updated because of how the form logic worked. In updated forms we only update the last mod date if actual changes occurs. This lead to the situation in which the user updates the asset media, but the renditions aren't updated: the root asset node stays intact!

There's a point in the existing logic though: when one restores the old version - it is the root node that was updated the last and should indicate that the variations need to be purged from cache.

Proposed solution: consider the latest of the two dates.