There is no input field for username, so I can't clear that. I cleared one or both of the password field (which was indeed autofilled) and the email field (which was set by the server in its value attribute), no change in behaviour.
...Ah! Found the username field, it was hidden behind the 'Change username' toggle. Clicked that, cleared the username field, cleared the password field (to avoid "The password confirmation does not match"), and now it works. I feel like a couple of checks on this behaviour (new username == old username, password correct, password confirmation empty) could avoid this unintuitive behaviour.
Tried the drag-drop-image thing with the plain text editor, and encountered no issues with it either.