These lines are caused by butt joints in the PVC
sheet foam. A filled Epoxy is used for the glue. This Epoxy is very stable,
thermally speaking. The PVC sheet foam will shrink slightly when exposed to
heat. This leaves the glue line standing proud.
Not a big deal if it is located away from the board's
major loads; or when the displacement is very slight. But placed in the
middle third of a board, and the outer skin buckled severely, and you have a
major stress riser that can precipitate snaps. Do take it up with your