Based on your description, can I understand that the meeting invitation email received by the delegator does not specify the specific person who is attending the meeting?
According to my tests, the meeting invitation email received by the delegator is shown in the image below, and the specific person who needs to participate in the meeting can be clearly identified in both red boxes.
If that's not the case in the meeting invitation email, you can share a screenshot with me.
If the remaining tenants don't have the issue, you can try re-authorizing the delegate permissions.
Moreover, you can also try the method Delegate Not Receiving Correct Calendar Invites - Microsoft Community
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