slice_single_volume_regions() sliced Precise Seam modifiers with
trafo_centered(), but a belt printer slices its layers with
trafo_sliced(): the belt rotation, any pre-slice remap and the lift off
the plate on top. On a belt print the modifier regions landed in the
unrotated frame, away from the walls they were meant to place the seam
on. Slice them with trafo_sliced(), as the support volumes and the seam
enforcers already are. It equals trafo_centered() off a belt printer.