Unable to receive email from internal users

Question

Rephrasing my question since I didn't receive any response to my original post.
https://learn.microsoft.com/en-us/answers/questions/1500102/unable-to-receive-email-from-internal-users

-We have hybrid environment with our on-prem AD syncing to Azure AD
-John Smith who used to work for our company now moved to another company with whom we have Azure B2B collaboration enabled
-John had member account in our Azure AD

-John was also invited as guest via his new company email

-instead of deleting his member account, we decided merge the guest and member account
-this is how we did it.

1. Unsync on-prem email (remove email, set sync to false)
2. Wait for next sync job
3. Confirm in azure user is COMPLETELY removed (check Deleted Users too)
4. Updated his on-prem email field  to match his guest email
5. Got the Immutable ID for his on-prem email
6. Mapped the immutable ID for his guest account in Azure to match the immutable ID from on-prem account

-ever since merging, John cannot receive e-mail from users at his old company, they all get a bounce back.

-John can receive email from external users and users from his new company