Hi @DrBone ,
Currently, MS Flow does not support the use of a Shared mailbox or Group email to create an email connection. The personal account is required here.
In addition, the features related to Shared mailbox available in Flow are the following two:

The points to be aware of when configuring are the following two:
- Need to remove the semicolon after the Mailbox Address.
- And the account used to configure the connection of the current trigger or action is a member of this Shared mailbox.
Hope it helps.
Best Regards,