I think an owners typical trip cycle use has a lot to do the frequency of FROM occurring as well; . so if you rarely to down to <1 on the battery level where the system kicks into its true hybrid mode and the engine is engaged running briefly to charge and the maintain battery back up to at least the <1 above absolute discharge then the FORM will come one much more frequently over the months. So its a combination of trip use, battery exhaustion cycling and time.
The only question that still bugs me with this engine design and application on these Hybrids is that by comparison, I used to let my old '95 Toyota Avalon (fantastic build quality and longevity on that one!) sit for a month or more, did not too frequent oil changes and never used fuel additives even here in Southern Canada in a only moderately cold winter climate, and never had a hint of engine trouble over 22 years, other than the two oxygen sensors which I replaced myself and a water pump replacement. My Odyssey which was a '99 EX model also had periods of no use and other than a the infamous EGR port clogging also never had any engine problems.
As mentioned in this description by Allpar in
Deep inside the Chrysler Pacifica hybrid system which describes the overall technical design of the Chrysler Hybrid system:
"The gasoline engine is also different; it uses the Atkinson cycle (like Toyota hybrids), which is more efficient but produces lower peak power. There is no exhaust gas circulation (EGR) or alternator; and the air conditioner is not driven directly by the engine (so it can be powered when running on full electric). Internally, changes including different camshafts, pistons, and spark plugs; the oil pan and intake are also different from the non-hybrid version."
-So can the different camshafts, pistons, spark plugs, oil pan and air intake system be much more sensitive to engine killing effects of stale fuel, stale oil and internal condensation than a garden variety ICE?
My theory is that Chrysler thinks so (based on computer modelling, destructive or real world testing or ??) so hence the programmed FROM to run the engine in gas only mode.