[2022] What decides if a room counts as separate or not?

When you are designating rooms (with the room toolbar open), the game will automatically add those designated blocks to the current room selected in the room list. If no room has been designated, it will create one. If you want to create a new room for every block you designate, you can hold ctrl. So, even though you've separated the room designations with walls, it will still think those are one room because you haven't designated a new room. What you should be doing is designating a new room (again, hold ctrl, making a new selection of area to designate) for each separate room compartment. You can add designations to a previous designated room by selecting it from the room list (when the room menu is open). An alternative way to select a room to add to it is by holding ctrl and right-clicking the block where a room is designated (with the room menu open).