The new Azure AD B2B guest framework for native mode currently does not support certain accounts, as per this article:
What features are not supported for Guests?
We are working hard to bring all Yammer functionality to the new B2B guests in Yammer. Below is the list of features still in progress:
Personal email, non-Microsoft 365 business email, and phone number-based legacy accounts – Users with Microsoft 365 Business email accounts can be added as guests. Other email domains like Gmail or Yahoo mail etc. will not be supported in this release.
So my question is: when will this be implemented? I looked at the roadmap and did not even see this listed.