(Not sure if this should be on the biology SE; also not sure what to tag this with so feel free to edit)
A quick Google search indicates that human bodies decay visibly within 3-5 days:
24-72 hours after death — the internal organs decompose.
3-5 days after death — the body starts to bloat and blood-containing foam leaks from the mouth and nose.
8-10 days after death — the body turns from green to red as the blood decomposes and the organs in the abdomen accumulate gas.
Given this, how is it possible that the bodies were visible two weeks before the Russian withdrawal? If they have been there for two weeks, how is it that the bodies haven't visibly decomposed?
A first guess is that the temperatures in Bucha are low enough that the bodies are preserved, but another Google search indicates that the temperatures are between 2C to 13C, which doesn't seem cold enough to preserve the bodies.