when using auto enchance on photos it darkens the photo but won't revert back.

When I use Auto Enhanced on a Live Photo it does what is expected.

As long as I do not allow the change to take affect, if I click the Auto Enhance Button a second time , it reverses the pervious action

Also, if you added some other edit actions to the photo then tap Edit on the right corner and you will see "Reset Adjustments" at the bottom of the edit list.

Good work 👍