Using New Outlook on Windows for professional communication and productivity
I FIGURED IT OUT! (May not apply to all)
A client had a Business Standard in their primary tenant and a Business Basic in the one he wanted to add.
The Traditional Outlook defaulted to the license key of the highest tier license/one authorized for the local app, and any subsequent accounts inharrite that "host license". The New Outlook is storing the keys separately. No inheritance. Meaning you have to downgrade to the traditional Outlook, or get the account that isn't App authorized upgraded to one that does.
MS is prob doing this to circumvent people from using the inheritance "hack" so people can't use it to decrease costs.