Here’s a news article about the problem. watchOS 5.1 Update Bricking Some Apple Watches [Update: Apple Temporarily Pulls watchOS 5.1]I see that Watch OS5.1 has been pulled by Apple as there’s been a few cases of the update ‘bricking’ some series4 watches during the upgrade.
That’s good. Otherwise you’d have to wait for a replacement from Apple.My Apple Watch Series 4 was not affected. The update went smoothly and quickly.
12.1.2 is for iPhone only.