Hi andy,
I also got this problem when testing on my Dynamics 365 App for Outlook.

This issue seems to only occur when tracking email in the Sent Items, and does not appear when tracking email in the Inbox.
And during the test, I also encountered the problem that the email sent in CRM is not synchronized to the Sent Items of Outlook.
I will raise a Microsoft support ticket to reflect this issue. I will post the follow-up progress of the case in this post.
Hope this helps.
Best Regards,
Lu Hao