Using New Outlook on Windows for professional communication and productivity
I believe I’ve found a solution. I think it works if you delete the email address and re-enter it. This is a dumb workaround, but it's better than the infinite loop I was in.
FYI: I figured this out b/c I'm using Outlook on my Gmail account. When I went to the draft folder on the Gmail website, I saw that my to: feel was occasionally empty. That made me think Outlook doesn't see the To: field content correctly.