As of now, PM requires a separate login for self-serve, the community, and the SIM ordering (Myshopify) pages. If and until these 3 are merged together, PM should definitely put up a note on each of these 3 login pages specifying that the other 2 logins will not work on that page.
This will eliminate a lot of confusion - and will only take one of PM's front end devs a sum total of less than an hour to make the change in all 3 places. This is not a big development decision that PM needs to make, and so there's really no argument in favour of not having it implemented right away.