Dear Matthias Thiesing,
Good day!!
Apologies for the inconvenience caused and please don’t worry, let’s work together on your concern and move towards a resolution path.
I understand your concern and to my knowledge the behavior you are encountering is not an expected behavior Save email to OneNote feature will be able to add the multiple email addresses but since you are able to UPN with onmicrosoft.com and your another private account is able to add more email addresses at Save email to OneNote feature, I would like to suggest you to perform the same action in the new InPrivate or new Incognito window to check the outcome.
If the same issue is happening even in the InPrivate or Incognito window, then I believe it should be a backend issue with your account where there is no any setting to change and the problem can be further investigated by related development team via collecting HAR logs because there is no any related service incident reported at the Microsoft Service Health dashboard.
I do feel regret for not resolving the problem and redirecting you to the related development team but the fix to your concern can be find out through the HAR logs and suggest you raise a support ticket from the Office 365 admin center, where the related team will investigate the problem further by collecting the HAR logs.
Reference: Get support - Microsoft 365 admin | Microsoft Learn
Note: To create a support ticket, you need to have admin privileges and if you aren’t admin of your Office 365 organization, please contact the admin of creating a new ticket.
Appreciate your patience and understanding. Have a great day!!
Best Regards,
Sophia