[Designincase]

I think some people may be pulling in BVS regarding kryptonian bodies.

Death of Superman

Superman was only dead for like a minute. His was brought back to life but left in a deep coma that probably would have killed him if his body hadn't been put into the regeneration matrix.

So until Kandor was brought back years later, they've never had Kryptonian body to study for decomposition. Maybe you can count Conner but his body was snatch up quickly by the Legion.

Also the black lanterns zombie face is just part of the Black Lantern process. Even people killed like a minute prior(Hawkman/Hawkgirl/Tempest,Captain Boomerang 2) had a severely decomposed look. They could also look normal until they wanted to up the terror.

It's not actually the bodies decomposed state prior to the black lantern process.