I found a way to reproduce your issue which is not bound to the version 7.0.
It's bound to the way walls are recomputed when you select the Recompute walls option in the room modification dialog box to change the appearance of the walls at room border.
In your case, the inner and outer walls at room border are slightly superimposed and this cause program think that it should recompute the outer walls too. Don't superimpose inner and outer walls or more simply unselect the Recompute walls option and it will work properly.