Exactly as stated - CTRL+M in LibreOffice Writer is no longer the hotkey to clear formatting after updating to 6.4, it will not react even when set to it, nor will setting it to alternate keystrokes clear the problem.
EDIT: In using LO, I’ve discovered that CTRL+M does work, but not when text is selected. It allows me to clear formatting while typing, but not to clear formatting on highlighted text. I don’t know if this helps diagnosis.
Ubuntu 18.04.4 LTS
I checked Tools > Customize > Keyboard. CTRL+M is blank. Remove Direct Character Formats is listed as CTRL+SHIFT+X, which I believe is the desired option (though I’m ready to be corrected on that).
I tested CTRL+SHIFT+X. No obvious response. No cleared format.
I set CTRL+M to be the hotkey for Remove Direct Character Formats and tested it. No obvious response. No cleared format.
Just in case, I tested CTRL+SHIFT+M, and it tried to close either the document or LibreOffice as a whole. The doc was unsaved, so I hit cancel. This wasn’t listed under CTRL+SHIFT+M in the Keyboard Customize window.
Searched here. My search-fu is not very strong, but I found what seems to be the exact issue here, including the CTRL+SHIFT+X issue. The solution listed there (renaming registrymodifications.xcu) was tried. File was renamed after closing LibreOffice. Reopened, but CTRL+M still did not work, nor did CTRL+SHIFT+X. CTRL+SHIFT+M still tried to close the window (I don’t think CTRL+SHIFT+M is relevant, but I mention it just in case). (After this, I closed LibreOffice again and restored the original registrymodifications.xcu file).
Gave up, posted this.
EDIT: Then I almost immediately realized an obvious step to take. I selected text, right clicked and clicked Clear Direct Formatting. The context menu worked, but the hotkey remains cold (I tested both CTRL+M and CTRL+SHIFT+X).