First you had asked if others had seen anything like this. And I will say no I haven't. This is the worst I have seen. So yes what you are describing is an outlier and I agree with JRB she should be able to appeal down to 3.0. But I also don't think it is proof that something went wrong with the algo let me explain:
In your initial post you said "Her partner was a seasoned 3.5C player that's been to 4 sectionals and 1 Nationals." And of course that is good information in understand how strong she is. However if I am reading Schmke's response correctly here:
You can have regular season matches before October 31 2022 if you have an early start or Fall league. Then play 2 matches in sectionals in June 2023. Those are matches, so they would be more than 2.
tt.tennis-warehouse.com
USTA did not consider his record for same gender play
at all. In other words when evaluating how USTA rates, you need to pretend none of what I quoted from you ever happened for her partner. You only look at his mixed doubles results. And even there I think you just look at mixed results that end with a .0 (7.0 8.0 etc) and completely ignore any mixed results where the team can end in .5. (like 6.5 or 7.5 etc)
The thing is we all know players can have good days and bad days. And of course your opponents and partners can have good days and bad days.
If someone plays 15 matches then sure it should sort of round out and the rating should be pretty solid. But that only happens if you include all those results! If 4 of those matches only count for one rating system and 5 only count for a completely separate rating system, and 6 don't count at all, then you can have some pretty substantial swings in what rating the same exact player might end up with. So her partner may have had a substantially lower mixed rating then his adult rating.
USTA does not publish any information of what her partners mixed dynamic rating was. TR is guessing this based on making guesses about his early opponents in mixed doubles etc. But unlike the adult ratings where TR will change their rating based on USTA EOY Adult rating, with mixed they often can't recalibrate. (because USTA doesn't publish Mixed ratings if someone has a Adult c rating) So those "errors" in TR (where "error" is defined as a deviation from USTA) will just keep piling on year after year. Piling error on top of error this way is why TR mixed rating may seem pretty far off from USTA ratings. That does not necessarily mean they are less accurate as to someone's true strength. It can be that the mixed ratings assigned by TR accord with reality better than USTA.