When i use the caret delete on multiple files and keep finger on the delete key then erroneous miscellaneous characters appear at start of most of the files.
Seems to happen when renaming more than about 10 files and is repeatable.
If I delete them 1 character, keydown-keyup on the delete key one after the other the it deletes OK.
See screenshots for example
I can't reproduce that here.
Please show us the full rename dialog so we can see which options are turned on.
Found a solution - I have halved the polling rate on my keyboard and initial tests show no errors.
What does the Macro Operation line in the dialog say after the problem happens?
(I don't think my keyboard has a poll rate option, but I've tried with the maximum key repeat speed and minimum delay and still can't reproduce what you're seeing, FWIW.)
Also after a glitch the undo button is still greyed out. (and also even if there is no glitch, but then it's not needed)
The undo button is only enabled after you click the apply button. Before that, nothing has happened to the files, so there is nothing to undo.