Recently migrated mailbox no longer receiving messages from external users

I have a mailbox that was working but not actively monitored in Exchange on-prem. After a recent staffing change, we migrated the mailbox to Exchange Online (we are hybrid still…) and now it is not receiving messages sent from external email addresses.

Mimecast (our online spam filter) showed some as being delivered successfully, but the most recent ones being bounced because of too many hops, as though there is a mail flow loop somewhere.

I could not find any rules on the mailbox to indicate it is forwarding somewhere else, so I wonder if there is an issue with our migration. I can confirm the mailbox now shows up as a “contact” in On-Prem exchange (like all other migrated mailboxes) and it only shows up in Exchange Online admin center. When I open the mailbox (because I have full mailbox access) I see the past few years’ worth of email, so I know it didn’t create a new Exchange Online only mailbox.

I also confirmed the user account has an Exchange license (E1).

Any other thoughts on what I can do to troubleshoot?

Quick update:
If I sign in to my legacy OWA site with my credentials, I get asked to redirect to Exchange Online.

If I sign in to my legacy OWA site with this newly migrated user, I get told the mailbox doesn’t exist or is not licensed (I’ve verified it is licensed for Exchange via E1 in M365 Admin).

This is also one of 5 mailboxes we’ve migrated using the new site, all of our previous migrations were done using the legacy Exchange Online Control Panel.