If your issue is specifically "Certain complications not showing up in Apple Watch App". Dealing with the new iOS 26 Version, this is most appropriate place to post rather than having 20 threads with the same title.
How my problem was solved with help from Technical support:
Preface: My exact situation: upgrading from an iPhone 15 Pro (iOS 26) to an iPhone 17 Pro as well as upgrading from an Apple Watch Ultra (watchOS 26) to an Apple Watch Ultra 3. Many Complications did not show up in the Watch app, (Apple Complications as well as third party), although they did show up on the watch itself. After many hours the problem did not go away by itself.
I would only take these steps after waiting a long time to see if they come back by themselves.
- Un-paired the watch
- Deleted the Watch app from the iPhone. (This does not remove watch backups.)
- Reinstalled the Watch app from the App Store.
- Re-paired the watch.
- Restored from the Original backup made before the upgrade to the new watch.(Not the new backup created from unpairing the new watch)
- After this many complications were still not showing up, but more showed up, then gradually more showed up until finally the app reflected what was on the watch, this took about an hour.
- Conclusions: this does seem to be some sort of syncing issue. For some reason, the process of syncing complications got hung up. If you have this issue, I would wait, in case it fixes itself. If a few hours go by, I would take the above actions proceeded by patience. Apple Care was very supportive; they shared my iPhone screen; they seemed as shocked by this behavior as I was. My guess is they will change things about the process to avoid this potential procedural glitch in future versions.
NOTE: This was one of the hardest Apple Hardware upgrades I have endured, both the iPhone and Apple Watch had serious issues and both upgrades had to be restarted from scratch.
[Edited by Moderator]