Tricky, but here goes.
Side paths branching off the main path are guaranteed to not be longer than 6 rooms. This was originally broken, got reported and was eventually partially fixed. It still happens whenever a side path hugs the border of the map.
The official stance is that the 10 room side path you are seeing is actually only 6 rooms long, it just happens to get extended by another side path that is 4 rooms long. This can happen repeatedly any number of times, so you may have to pass through several dozen rooms to reach the end. However, when counting rooms and reaching 7, you have to continue counting with 1 instead. That way you are always within the guaranteed length, so nothing has to be fixed.
To sum it up:
You can also read up the rest of the story there, it’s actually a very creative approach to redefine math. You’ll notice that the bug report is still open after a year. Apparently the official reasoning was considered too controversial to continue, so the bug report was rather left to gather dust instead of actually addressing the issue.
This may count as “spreading lies”, so this post may spontaneously disappear.