Deleted the guest user from Azure AD by searching for JaneD@domain.tld. I suppose the lesson is that these Guest Mail Users will trump your contacts list, the displayname in the email header and just about anything else. Hope this helps someone.
One possible reason why the displayname was...
Configuration (may not be relevant to the problem):
Outlook 365 v1712 b8827.2131 click to run.
I have O365 account as primary with several shared mailboxes added. Primary includes online archive.
Secondary account is Exchange 2016 on-prem with cloud archive (hybrid).