I did the testing in my lab and get a different result which looks more logical:
Then Admin sends email to group1, User C.C approve this email:
User A.A in group1 could receive this email successfully:
Then Admin receive an email which said cannot send emails to Group2:
So, if your group configuration are correct, this email will not be accepted by group2. I would suggest you double check about the configuration.
If the response is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.
Any update about this thread now?