Not All Calendar Events Showing On Watch

Then at this point I would recommend that you use the Get Support link at the top of the page to contact Apple Support. If you have an Apple Store close by, then you can make a Genius Bar appointment and have Apple examine the devices (iPhone and Watch). They will explain your options. Genius Bar Reservation and Apple Support Options - Apple

I have not experienced any problems with my calendars appearing on my Apple Watch Series 6. Do you mean it has been a long-term bug for you? Have you checked on your iPhone in the Watch app to see what settings you have for calendar? Are both your iPhone and Watch have the latest operating system updates installed? What version of iPhone and iOS do you have? What version of watchOS do you have. The latest iOS is 15.5, and watchOS is 8.6.

Have you tried powering the Watch off and back on again? Have you tried powering both devices off, then back on, making sure the iPhone is powered on first? If that does not help, then try resetting the Watch. Press and hold the Digital Crown and power button together until you see the Apple logo and then release. The Watch will reboot. Test. If that does not help, then unpair your devices, wipe the Watch, then repair the devices using your Watch backup.