Hi David,
According to your description, I have ran a test by referring to your steps and the same issue occurred on my Outlook for Mac 15.6 and nothing happened to my Windows Outlook 2013.
After lots of testing and research, unfortunately, this is an expected behavior of Outlook for Mac client, and I have found no Official documents that can provide a solution to the issue. Sorry for all the inconvenience caused.
Your understanding is highly appreciated.
Best Regards,
Mouran