I'm a big skeptic so dont be offended by the IFs
Take a look at the attached page from the manual the grate system is simple and the best way to fix a problem is to start at point A
and examine and recheck the important variable's. If your draft readings are correct as stated I would think that eliminates that but when where these readings last checked and verified
Do it again if not within the last few days just to be certain and if there is any uncertainty about what you are doing get some help with it . To most it would appear to be the most likely cause of the problem.
If all that checks good I would remove the large cast iron grate holder and inspect for warping or some other defect that is preventing the grate plates from fitting correctly and laying flat all around the edges of the plates, also check that the the gasket underneath is in place and sealing. Was the bolt retaining the grate holder in and tightened ?
Since this is not a usual problem we got to look at the odd stuff I'm wondering if while there is enough airflow coupled with a strong draft to support good combustion but that the volume of air being moved by the combustion fan up thru is not enough to create a slight cooling effect to the underside of the grate plates . Also if the grate holder passes a visual inspection I would head to my Dealer for a side by side comparison of a new one .
Its a simple system and something is not right so start at point A
and eliminate the possibilities