Only within the Microsoft ecosystem could this be deemed "solved".
There's still no Flow trigger for certain emails in a shared mailbox.
We currently receive a variety of emails to a certain shared mailbox and would like some of them to be automatically added as Planner tasks.
As identified, there's currently triggers for "when an email is flagged" (but this is purely for individuals' mailboxes) and for "When a new email arrives in a shared mailbox".
There is no way (it seems) of triggering "when an email is flagged in a shared mailbox".