Fluctuating screen time mystery on a kid's phone

My daughter appears to have figured out some way of getting around screen time parental controls and I've done a lot of detective work but still can't figure out how. I recently noticed that her screen time on the two apps she uses the most had gone DOWN — in fact was suspiciously low. So I began monitoring the screen time on those two apps (iChat and Snapchat) and was surprised to see that in the course of the day the screen time on those would fluctuate — she would approach her limit and then the next time I looked the time would have gone down. Also, notifications for those apps would increase and decrease throughout the day.

For example screen time might show 15 minutes total in the morning, then 30 minutes total by afternoon, then 5 minutes by evening!

I changed my password and updated both our OS to the latest.

I read a very helpful description here of how to prevent a kid from changing the time zone to foul up screen time, but when I followed those directions I discovered that the ability to change time zone from her device was in fact disabled.

Any apps she could have obtained through the app store would have to be approved by me, so that's out.

Any ideas what could be going on here? Interestingly, the TOTAL screen time does appear accurate. I think it's just the individual apps that are being overridden ... but how???