And just to check, when you use System Settings > Users to try to change the currently logged-in user’s password, you’re prompted (in the highlighted area) for the root user’s password, not your own?
@AlbertoSalviaNovella If the OP here is getting prompted for a root password, then that’s confirmation of this bug being reproduced in Plasma 6: 378984 – Root password required for user Password change - if they were only getting prompted for their own password, that would be the desired behavior.