Similar experience here. We have attempted to invite guest users (that already have a Project license via their home organisation), but their existing Project license isn't recognised. We have reluctantly purchased new Project licenses for the guest user accounts (so they now have double licenses - one from our organisation, and one from their home organisation). Some of the users are able to view/access the projects. But a couple of users are unable to access projects, despite having valid licenses assigned to their guest user accounts.
This is a terrible implementation of sharing projects and collaborating with external users. I do not understand why Microsoft simply don't unlock the ability to share a project via a URL (exactly the same as it works with Sharepoint/Onedrive/Word/Excel/Powerpoint/etc). Is is painfully apparent that Microsoft has deliberately locked this essential feature (which you get by default with the vast majority of Microsoft services) in an attempt to try to get more people to purchase these overpriced licenses just to be able to show a project to another person. You have cashed in on 4 "guest licenses" from us, half of which doesn't even seem to work. But we are getting closer and closer to stop using MS Project. This is simply not working