Why is my Apple Watch Ultra calendar not syncing with my iPhone?

I have an Ultra Watch version 1 with the latest Watch OS 11.0.1. Recently, the calendar on my watch stopped syncing with my iPhone 13 Pro running OS 18.0.1. I can't get the calendar to re-sync.

I have tried a few of the tricks I read online, such as:

• On my iPhone Watch App -> General -> Reset -> Reset Sync Data.
• Changing the watch face to one that does not have the calendar on it.
• Doing a hard reboot of my watch and phone.

Nothing is working. Any advice would be welcome!

[Re-Titled by Moderator]