Settings were reset when updating the watchface

A few users complained to me that the watchface settings will be reset after updating. A few were upset and angry. I can reproduce it on my FR645, but not FR255. What could be the reason for that? 

This has been raised in the Discussion, but I'd like to know the exact reason, and maybe a possible workaround, so I can explain it to my users.

Some related discussions:

https://forums.garmin.com/developer/connect-iq/f/discussion/356891/settings-lost-after-watchface-update/1716707#1716707

https://forums.garmin.com/developer/connect-iq/f/discussion/4890/how-to-retain-settings-after-update-of-a-widget-app/32636#32636

https://forums.garmin.com/developer/connect-iq/f/discussion/5049/seeting-property-value-reset-when-updating-up

https://forums.garmin.com/developer/connect-iq/f/discussion/6231/app-setting-are-lost-when-widget-is-updated