Hello @remba87, I have experience this issue before. I am not sure what causes this redirect to occur, but the user should still be able to access as expected if they go to the proper link.
In scenarios when they are directed to the root user IAM sign-in page, have them go directly to the quicksight sign-in link with your account alias attached at the end. It will look like this:
https://quicksight.aws.amazon.com/sn/start?directory_alias=yourAlias
Then, they should be able to bypass the IAM sign-in page and update their password. Let me know if this helps!