You cannot be serious

This is a crazy one even for the uber-craziness of the Mandela Effect!

First, the type of phenomenon here is similar to the Razzle Dazzle ships or the second longest wall in the world, i.e. it's something you'd expect everyone to know about but hardly any do. This kind of knowledge can happen in any field, but some, like this, are so startling it seems very unlikely it can have been missed by so many people for so long, hence the idea it suddenly appeared from nowhere and so is being classed as a Mandela Effect.

The painting does indeed show God flying away from Moses baring his buttocks.

It definitely is God, because the image forms part of a narrative and is next to an identical one showing God creating the Sun and the Earth. And it's clearly not a slip of the robe or similar clothing malfunction, since there actually is also a reference to it in the bible.

Exodus 33

Michalangelo was painting a literal section of the bible, when God was talking to Moses: "And I will take away mine hand, and thou shalt see my back parts: but my face shall not be seen."

Credit goes to MoneyBags73 for this: