Issues Sharing Reports With Users Outside Our Organization

nickyvv & V-lianl-msft, we finally figured it out. You both pointed us in a direction that helped us identify the issue.

The key take away was that a guest account is created when an invite is sent from Power BI. I tested this by sending invites to my hotmail and gmail accounts and sure enough as soon as the invite was sent you would see a guest account created in Azure AD.

However, for the users in question no guest account was being created. Without getting into all the details, these users used to work at our company but now work for a new company. Their AD accounts are still active and their AD accounts have been updated with their new email address. So when the invite was sent to the user in quesiton with their new email address, that email address resolved to our internal Azure AD account and as such didn't create a new guest account.

So in order To Resolve The Issue we had to prevent these AD accounts from syncing with Azure AD. Once we did that and the sync was completed we were able to send these users an invite and everything worked as expected. 

Again, thanks for the input as it utilimtally pointed us in the right direction.