If you go to settings...accounts...work/school....can you post a screenshot of what shows up there on the computer(s) that are logging in with the work/school account?
I'll have to snag this next week, probably. I can say that since this is the owner's own office machine, on that machine I am not using any "secondary local" account to log him in. We're going straight in with the one and only account that was ever used on this box.
The problem with the AzureAD side of things is right now it's with GoDaddy and timing means that defederation of that tenant would have to have happened first, were this to be graceful, and it hasn't. We just started discussing this today in terms of moving toward M365. But I would also certainly hope that I do not need to N&P the other machines, which are working just fine and everyone's happy with, to get them joined up with the M365 world that will be created for the business as a whole. It's constantly being said that local accounts can be logged in to M365 that, for lack of a better way of putting it, is "of another sphere" and by that I mean it's not someone's M365 Personal or Family account, without needing to do anything to them. On the other two non-owner machines I have set up local accounts and those are what the end-users of those machines use to log in. The business owner's account exists on those machines, too, but, as I noted earlier, when we were setting these up earlier the same email address was in use for both work/personal and, at that time, I went the "personal" route for those.
I would think that it's possible for anyone, including me, to log in to a given M365 tenant and use the services associated with it if I have the correct credentials regardless of where I might be or what computer I might be using. Maybe I'm entirely wrong, but I think of the M365 ecosystem as roughly equivalent to the Google ecosystem in that once you're logged in with any given Google Account, you have access to the mail, drive, calendar, voice, etc., etc., etc., for that account. And you can log in to that account from virtually anywhere. If M365 doesn't allow me to "join" anyone to an existing tenant that I wish to add, then I need to learn what has to be done. I've had offers from several members for private conversations on M365 and it looks like the time is coming, very, very soon, to start having those.