AUs are confusing and unpredictable. One of the things you're looking for is predictibillity. Run whatever tests you want on primary user cards first. Once you're satisfied with your primary card testing, play with AUs.
This may take a while if there are a number of things you want to look at. Take your time, and resist the tempation to try to test two things at once. If this takes a few months, you might want to check in at true AZEO with a major card at some point. That's just to see if anything other than what you're trying to test has changed.
Was this FICO 8 on TU?
I can't argue the datapoint as you say, seems well isolated and controlled.
Just for giggles on the myFICO TU monitoring service you should get reason codes on "Your Score Changed" alerts; did you get any of those alerts during this process and if so by chance do you have the reason codes for when your Carecredit was the only revolver with a balance?
Weird, I would like to see the test redone on another store card (and not an AU as already metnioned) but certainly looks concrete.
"Yes I do have the reason codes from MF and they are simply balance changes and score changes"
What I mean is look at the score charge alert's for TU, if you expand it out (least in the web interface) it comes with reason codes.
You should absolutely get a reason code for AZ in my experience, the question is if the reason codes are different with just the Carecredit reporting a balance vs. a national bank card.