(Noticed in v2.91d, also reproduced in v2.93)
When editing a format string, in rare cases you are not given the actual string to start with, but an old string from the string history. I noticed this when I accidentally used uppercase, but I'm not sure if this issue is limited to uppercase/lowercase.
To reproduce, do the following
Open Action groups, make an Action > Format value and type Field: ARTIST / Format string: Ringo STARR. Click all OK's.
Open the Action again and edit from Ringo STARR to Ringo Starr. Click all OK's.
Open and edit again and you'll see the discrepancy between Starr and STARR
I also tried to reproduce by misspelling Paul McCartney to Paul McCartnay, but everything was well, so maybe the bug is caused by uppercase/lowercase? It seems that variations of uppercase/lowercase are not saved in the string history.
Also, I wonder - does the edit string come from the history? Shouldn't it come from the saved Action? I mean, you normally have much more complex format strings than in my example above, and any history error would not be so easy to notice.