Hey @Nessy,
It is really not an issue if you are creative-building. If you are chiseling in survival-mode it is really handy. The /roomregdebug command is used to see if the game has 'reg'istered a space as a room. In winter, if the space is not registered as a room, it is coooold. There is a fair bit of conjecture as to what the limits of room size is. The suggested max size of a 'room' is 7x7x7. But I have had larger spaces recognized as a 'room'. The general rule is a block must have a minimum of 50% depth to be recognized. Slope- chiseled blocks usually don't meet that criteria. So a custom sloped roof needs a ceiling under it.
The chimney and chiseled glass in this gave me fits. Its hard to see errors in glass.
There are three command arguments to the /roomregdebug command.
'hi' turns on highlighting (all green if the room is registered. Red it not)
'unhi' turns off highlighting
'list' shows a count of possible room dimensions?
It seems to be chunk-based, or at least chunk-sized when it fails, to judge by the second screenshot.
My troubleshooting consisted of blocking suspected areas until I figured out just where the problem was. In one case I could not find it and just redid it. One tiny nick in an already thin block can do it.
I did not stay in a Holiday Inn last night