Company Portal on Windows 10 Installs OK but then reports Error installing itself

OK - So removing the assignment from the device assigned system installation for Company Portal stopped the error. I then created a user assigned installation for Company Portal using Install behavior = User, but this is failing to install on a newly enrolled device/user combination - it just says "Waiting for install status" on the machine managed apps, however Win32 apps, M365 apps, and Edge all installed successfully. The Intune Managed Portal just shows 3 users have got the company portal out of around 40 in the pilot group. Where do I begin hunting?

Edit < I just had a brainwave - the users have a very short SAM Account name and since the devices are Hybrid Joined, they are probably using that to sign into the devices instead of their much longer Synced UPN which is luckily the same as their email addresses (so we can tell them to use email address to sign in as they would not know what a UPN is). I am asking the new one to test by signing in with their UPN now.>

Also - the image of the company portal showing its own app in a failed to install state which I put into the first post above, comes by clicking on the error notification in the top corner of the company portal itself rather than having it assigned as available to the device or user.