Hi,
I am no expert at the Office365 suite, so my logic might be flawed and my answer might not help , i will be sorry in advance and hope that someone else can give you a better answer.
Would it be a approach for you to create a new user with a email license and convert shared email adresses to aliases instead, which i can confirm works in Zammad?
But below is some threads covering your issue.
This previous thread reports some issues with shared email adresses.
https://community.zammad.org/t/office-365-shared-mailbox-setup/4476
And here is another thread which suggest that shared email adresses is not the “correct” approach.
https://community.zammad.org/t/m365-oauth-access-to-shared-mailboxes/10224/10
Best regards.