The issue I think Is with the quality of the coating. The placemats giving he worst results had severe rippling in the coating whereas the placemats the had best results had smoother coating. Holding the mats upto the light you could see the difference. Obviously not scientific proof but my conclusion. Just did 6 placemats today full colour and They came out great, my batch was from xpres who are normally very good but mixed quality this time. I need to order more but will contact them about the this prior to ordering. Who did you buy yours from?
As for the boards bending I solved this buy dampening the back or laying them on a damp cloth (not soaking) with a very heavy object on top for an hour. The boards bend because moisture is drawn out of them when pressing. Its trial and error to suit.