When using a chiseled block, the room does not count as a "room", and your food rate still increases when cold.
I'm not sure how difficult this one is to manage, given that chiseled blocks can be any shape. I ran into an issue with a block I chiseled by mistake, but didn't actually modify it. Because the block looks identical to non-chiseled blocks, it's very hard to track down.
If not possible to calculate rooms with chiseled blocks, maybe an overlay/dev tool to display valid room-blocks could help. Seems like slabs are valid blocks.