I’m just going to leave this reply in case anyone in the future, like me, came across this same baffling problem. The solution was horribly simple yet hard to pinpoint at first.
For me, I had the same symptoms as above – editing a page or post caused the permalink to change to my username… completely baffling. I tried all sorts of forums/fixes until I tried another browser. The new browser wasn’t affected! So I looked further… it turned out to be my freaking password manager extension entering my username into the hidden permalink field without me seeing it and thus updating the permalink to my username for that website EVERY time I updated a page. Painful!
The culprit for me specifically was the “PasswordBox” Chrome extension.
