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[sam (Sam Saffron)]

@justintime

Every time someone clears their cache and re-logins into Discourse, it wipes out their username and resets it to the word “user” and then a number. If I fix the username manually, on the next log back in, the username is reset to “user1” again.

I have a feeling that somehow you have you SAML implementation overriding usernames, so when they re-auth, we take the name provided by SAML.
I would check saml_clear_username or auth overrides username settings.