Why does my Apple Watch not accurately record my stand times?

Besides standing, you have to be moving for a minute at the same time. If you stand perfectly still, you will get no credit toward your stand goal. The first (and simplest) thing to try if something isn’t working right is to restart your Apple Watch and its paired iPhone. If necessary (device not responding), try a forced restart. Both methods can be found here:

How to restart your Apple Watch - Apple Support

Restart your iPhone - Apple Support

If that doesn't help, the next steps would be to unpair and pair your watch again and restore from the backup just created. This should result in no loss of data. It deletes all temp and corrupted files and re-indexes the file system.

Unpair and erase your Apple Watch - Apple Support

Set up your Apple Watch - Apple Support

Technically, you Apple Watch arm needs to be moving in a manor associated with swinging your arm while walking.

You can be laying on your back, but if your Apple Watch arm is swinging back anf forth, the Apple Watch will count it as standing if you do it long enough.