Regroup Rhythms changes spelling of notes that should not be affected
There is no conceivable reason Regroup Rhythms should affect this:
But it does, transforming it to this:
Thankfully, Respell Pitches (which in 2.1 is not in the same menu as Regroup Rhythms) puts it back.
See also #196771: Regroup Rhythms deletes various elements, even more lost on undo. I feel bad that I didn't do enough testing in the RC to catch these sooner.