As you are using Exchange Calendars, try some or all of these (which have 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 has not yet been reported back on, so you may prefer to skip it - especially if you have followed all of the previous steps:
Remove the Exchange Calendar from your iPhone, check it for problems (and resolve them), then replace it:
After making changes:
- On your iPhone, in the Apple Watch app, go to: My Watch > General > Reset > Reset Sync Data (process runs in the background - allow a minute or so).
- Restart both your iPhone and Apple Watch (to restart Apple Watch: Press and hold the side button until you see the Power Off slider; Drag the slider to turn off completely; Afterwards, press and hold the side button until you see the Apple logo).
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. Use the Apple Watch app on your iPhone and choose to Restore from backup when setting up again (this will retain most data and settings):
Unpair your Apple Watch and iPhone - Apple Support
Set up your Apple Watch - Apple Support