I have an iPhone 12 Pro (iOS 17.1.1) and Apple Watch 3 (watchOS 8.8.1). I have been having this problem since late September, and it has been concurrent over multiple updates on my watch and phone.
I am using the default mail app with a Comcast and a zoho email account. I have it set to fetch every 15 minutes. I am still receiving emails and they are in the mail app, but I am no longer receiving notifications just when I’m wearing my watch.
If I am not wearing my watch I get the normal notification mail preview on my phone Lock Screen and everything acts as normal. The second I put my watch on I don’t get notifications on my watch or phone Lock Screen.
I have tried both “mirror my iPhone” and “custom” under the Mail settings on my watch. I also have all the notification options turned on on my phone, and also have alerts, sounds, and badges turned on for that specific account.
I have unpaired my watch and set it up as a new one. I have also tried restarting my phone and watch after changing settings in hope it will fix it. The only things I haven’t tried is restoring my phone from a back up, and deleting the mail accounts and setting them up as new ones.
This seems more like an issue with watch settings, but I don’t know what else to try. Has anyone else experienced this issue, or have any suggestions?
One time I had issue with getting mail notification on my watch, I have to go to the watch app on iphone > Mail > Accounts and login to my email accounts. Some of them shows "Sign in required" for some reason. After that the mail notification works perfectly on my watch.
Not sure if it's the same as your issue though.