The rule is that Walls shorter than 6 feet (approximately 1.83 meters) are not shown as Cut (ie. thicker lines), even if they intersect the cut plane of the Plan View – they are shown in Projection UNLESS they are attached. If they are attached to something (like a Reference Plane), they can be shorter than 1.83 and they will correctly show as Cut in Plan.
As soon as you set the wall properties to Unconnected and Detach its top, it will show in Projection mode again, if it is shorter than 1.83m.
How it affects Cut vs Projection in Plan