Thank you @EJ Marmonti for posting your query on Microsoft Q&A.
From my understanding you are trying to reset your password for MYSQL flexible server using the Azure portal. You also mention that you need to take a mysqldump so you want to use the admin account
Could you please make sure some of the admin privileges was not deleted? Are you using the admin account to reset password?
Could you please try to use CLI to update the password?
az mysql flexible-server update --resource-group myresourcegroup --name mydemoserver --admin-password <new-password>
Please let me know if the issue still occurs.
Regrads,
Oury