Using classic Outlook for Windows in business environments
I had the same problem and found reproducible conditions and a solution.
In my case, it was due to the difference in the display scale of the monitors.
If you are using multiple monitors, check to see if the display scale of each is the same.
If they are not, set them both to the same display scale.
(Right click on the desktop, "Display settings", "Size of text, apps, and items on this display")
For example, if you have two monitors with different display scales and the appointment window is opened on a monitor other than the one where the main Outlook window was first opened, you will be prompted to save the appointment even though you are not editing it.
I believe this is a bug.
Hope this helps.
Ken