Why do enemy bodies sometimes fly away and disappear after a death cinematic?
It seems your PC may not be able to handle either limit for corpses or the FPS rates. For the FPS it basically just throws the bodies through the ground or walls during the cut scenes to adjust for that problem. For the corpse/item limit it just counts the number of items in an area and removes those that can be removed that won't affect your character or quest. This has also happened to me several times especially near water or structures that have too many pathways and content. It also happens in other games with the same engine (namely Fallout 4).