The approximate mechanics in vanilla were, that you were allowed to get out of combat on the initial turn, if no one noticed your attack. This usually happened in empty rooms with only a single enemy in there i.e. there was no one else to notice the attack. Usually in full rooms in vanilla you had to wait for a turn. Obviously I don't know the exact specifics.
In RE I can get out of the combat in a room full of turrets on the initial turn.
SLOT02.zip
RE -> https://streamable.com/my99un vanilla -> https://streamable.com/z4s6bw
The approximate mechanics in vanilla were, that you were allowed to get out of combat on the initial turn, if no one noticed your attack. This usually happened in empty rooms with only a single enemy in there i.e. there was no one else to notice the attack. Usually in full rooms in vanilla you had to wait for a turn. Obviously I don't know the exact specifics.
In RE I can get out of the combat in a room full of turrets on the initial turn.