Send email on behalf of shared mailbox, using connector w/ delegate permissions - "Not authorized"

I am working on a flow that, at the end, sends an email from a shared mailbox.

The connector this step is using is an account that has delegate permissions on the mailbox. 

Here is the send-as delegates on the shared mailbox:

So, theoretically, this step should use this connector (which has send-as permissions) to send the email from the shared mailbox, yet, I am getting this error:

Any thoughts on what could be the issue?