it seems to have become more frequent recently.
i've been experiencing the same on firefox and i've also heard other people report the same on firefox, which happened around the time of the firefox 129 release. i didn't see anything noteworthy in the release notes though that'd explain this. it seems like it might be related to enhanced tracking protection and cookie isolation.
account names cannot be changed.
you can only change your display name, which is available in the settings.
whether display names or usernames are shown depends on the interface/client and user settings where available.
the only way to change the username is to create a new account.