I'm wondering if the issue is the amount of devices registered to the account is hitting a limit somewhere else (like Intune). You shouldn't actually need to be a Global Admin to join a Windows 10 PC to an AAD domain - any normal user can do that.
I should have said that I am using AAD Basic, so Intune is not involved. I only tried another Global Admin in case there was some limit per account and wanted to be sure that it was not permissions related, in the Azure console I have not set any restrictions on who can join and set the number of devices to unlimited. The existing joined machines were joined by more than one account. If there is a limit then it is very low...?