Raw veneer has what I call plus and minus grain. The tight/good side has some high grain and the back side has some low grain. With good quality veneer, the sides are really hard to tell apart. Worst case is lots of high grain on the front and "deeply pocked" grain on the back. The only maple I've gotten that had significant +/- grain was some highly figured western.
What was the moisture content of the veneer and substrate before you started, had they both had time to acclimate to the 45 to 50% environment? Since the RH went down and you have puckering, instead of splitting, the core had to have shrunk more than the veneer. Either the core had a higher moisture content before pressing, or it expanded more/absorbed more moisure during pressing.
I have no idea how heat affects the process. Is a heated platten in direct contact w/ the veneer, could it drive moisture out of the veneer and into the substrate?... Is the temperature enough to affect the bond between paper and veneer?
In my experience with PVA cold pressing at >= 20hg for 1/2hr with 1/4" MDF cover sheets, splitting would be more likely. My single side layups always dry flat or cupped on the veneer side, i.e. the glue/veneer contracts more than the substrate.
I wouldn't think a 15% change in RH would be enough to cause that puckering unless there was, at some point, a significant difference in moisture content between the materials and/or there was a pretty weak/non existant bond between two of the materials.