As you are using Exchange Calendars, try some or all of these (which were discovered by and worked for others):
- Archive your Exchange Calendar for all items before today.
- Remove / delete all old inbox messages about calendar invites (accept and decline notifications).
- Create a rule in your Outlook Mail account that moves all future accept and decline replies - immediately upon receipt - into a separate mail folder that does not sync with your iPhone and Apple Watch.
This potential solution has also been tried, but not yet reported back on:
Remove the Exchange Calendar from your iPhone, check it for problems (and resolve them), then replace it:
After making changes:
For a more thorough reset and refresh, you could also unpair and re-pair your Apple Watch. Do this via the Apple Watch app on your iPhone, which takes backups of your watch automatically including a fresh one when you unpair via the app: