After updating my Mac Studio to macOS 15.6.1 both monitors are waking up from sleep

Well whatever the cause it’s not the latest 15.6.1 update. All it did was fix a security exploit that may be being used in the wild, emphasis on ‘may be’.

Try restarting your Studio in Safe Mode which can sometimes correct a glitch that occurred during an update.

Start up your Mac in safe mode - Apple Support

Try searching for this issue to see if other users who updated Lightroom Classic are seeing it too.

Try toggling the sleep options to a different time and then toggle back.

In the end all it takes is for one bit to switch from 0 to 1 or 1 to 0 for a problem to rear its ugly head.

It is possible that some app is running creating the issue but with the last few OS changes users have been having issues with the sleep feature. If you go to the lock screen menu you need to "turn display off when inactive for TBD minutes"

then in the energy menu click on "prevent sleeping when display is off"

.

Test that and see what happens

I see so many users with issues using sleep and no resolution. I recommend to not use sleep. Just have display turn off after a set time.