Hi Lew,
If a mattress is soft enough for side sleeping then there is always a good possibility that the mattress could be too soft for stomach sleeping (which needs a firmer thinner comfort layer). Of course you have compensated for this with your zoning approach.
I would also consider that particle board is more subject to bending and sag than either plywood or other types of wood but this would also depend on the thickness and the span distance. Either way … it could make a difference for someone that was more sensitive.
A solid surface under your mattress would also increase the likelihood of moisture retention in the mattress (see post #10 here) and if you are also using a waterproof mattress protector this could lead to the retention of moisture in the mattress. One of the “symptoms” that also points to this as a possibility is that when you rotate the mattress it’s OK for a few days before the issue re-appears.
Of course all of this speculation may lead past a point of diminishing returns and it seems you have mostly solved the issue with your zoning system anyway but it may be interesting to check a little further on a possible humidity connection by leaving the mattress unmade and uncovered during the day for a while to let any moisture trapped inside (by either the platform or the mattress protector) evaporate more easily to see if this makes any noticeable difference.
Phoenix