Apple Watch screen is darker after watchOS update

Restart your watch and see if it makes any difference:

Hold the power button until you see the SOS screen. Tap the power icon in the upper right hand corner, then swipe to power off. Wait a minute, then press and hold the power button again and hold it until you see the Apple Logo. Let go when the Apple Logo appears. Enter your passcode when prompted.

If turning the screen brightness all the way up nor the restart helps, you'll want to schedule an appointment at your Apple Store Genius Bar to have your watch tested.

I've seen no difference in the brightness of the screen on my Apple Watch Ultra after updating to the latest WatchOS 9.5.1, which was released about 3 days ago.

I have also noticed a display brightness difference in my Apple SE watch after the last update.. it gets dimmer the darker the room is and lighter the more light you have.. I have shut it down and restarted it but didn’t get better making watch very hard to see and read in darker places!! I didn’t have this issue before the update..

I'm very glad it helped you. Please remember to do that procedure after EVERY OS update, for all of your devices. For iPhone and iPad, you hold both the power button and volume up buttons at the same time till you see the swipe to power off.

You're very welcome!