Discussed with Sybren in the blender-chat.

Long story short: the fix seems fine, even for 2.91.

Longer story.

The fix is fine, but while looking into it some technical debt has been discovered. For example, it would be more clear to introduce BKE_driver_allow_reevaluate() instead of direct manipulation with the flag. Makes it more obvious and explicit what the intent of the code is. The comments might need to be updated as well.

Just to be clear: no need to address the debt now. Writing it down for "the protocol": so that it is listed somewhere more permanently than the chat. Eventually the animation/depsgraph teams will get into it :)

I also think the patch is fine for 2.91. The current code in 2.91 will behave differently depending on order of f-curves, and even if there is some problem with the code it means that artists files will stop working at random. With this change any possibly hidden problem becomes way more easily reproducible. And we still have 2 weeks to catch issues.
This is worst case analysis. The code seems to be correct with this patch.