Stock surfacing issue

Lacking a planer, I have been using my CNC to plane my stock before cutting. I have noticed a consistent issue where the middle of the stock is thinner than the ends of the stock after surfacing both sides. E.g. the stock looks like )( when standing up straight. Measuring with calipers, it looks like the middle of the stock is about 0.7mm thinner than the ends of the stock after surfacing.

Has anyone had success using their CNC in this way to mill stock? I have watched several YouTube videos on the topic, and I am copying the usual methodology.

Have you surfaced the wasteboard recently? Youll want to do that seasonally if the humidity level swings much where you livd because mdf will absorb and release moisture and change shape, possibly bowing up in the center. Also making the wasteboard as rigid as possible, will help it move less as well.