that's not ship lap. that's a solid plywood board. someone didn't know what they were doing.
tell them you want a tongue and groove wood,shiplap or MDF panel evenly cut and spaced. ,,,then show them a picture
it can be done w/MDF cut like this, and thinly spaced.
and btw,,,,the framing on the outside should be done over the horizontal boards so you don't get those tiny gaps between the two.
Look at these pics for examples. (pic #1 and #2 are the closest to the way yours is being done)
in fact, show the bozos all of these pics and see if they understand.
Q
Classic gray
Q