I was experimenting with sheet metal design methodology and noticed that when I used one procedure, TC appears to give up the bend centreline. I formatted a box as surface from solid, then facet-edit-deleted one side. I chamfered the corners of the open side, then filleted them. The chamfer offset was greater than the radius of the fillet, which causes this effect, I think. I facet-edit-deleted all of the chamfer and fillet faces except for the ones I wanted to unbend, then shelled and unbent what was left. If you look at the corners of the unbend result, it's left angular cutouts with apexes that locate the bend centreline.