As announced in the thread about Sweet Home 3D 5.5, doors and windows now always cut out both sides of straight or round walls when they are parallel or tangent to them. If you don't want a door to behave like this, you'll have to reimport its model without checking Door or window option (or set its [font=courier new]doorOrWindowWallCutOutOnBothSides[/font] property set to [font=courier new]false[/font] in a furniture library, but that's more complicated).