FS730V Backfire under load

I'm kind of stuck right now, I have a Kubota z421 with the FS730V with 192 hrs. Everything was running smooth until about 4 months ago, when the engine began to backfire whenever the blades were engaged (as soon as the PTO was disengaged, the backfiring stopped). I will add that the mower was left outside overnight one night and we got some light rain. I know this usually means there is something amiss with the air/fuel. I changed the fuel filter, new spark plugs and fresh gas...didn't fix the issue. I thought water may have crept into the tank from when it was left out. At a loss (and no spare time) I took it over to my local Kubota dealer. First they said the PTO clutch was bad (due to yard debris) and that this was causing the backfire. After being told it was fixed, I brought the mower home with the new PTO clutch, fired it up and boom, the issue is still there. Took it back over to them, they removed the heads, to check the valves since I was told if those are out of spec, it could be causing the issue. Valves were all good, so they kept digging until finally saying the piston rings were bad, which was the cause of this problem.

Got the motor put back together and finished installing today, but this is where defeat sets in....fired up and running good, engage PTO.....instant backfiring! Carb was fully cleaned as well. I can drive it around all day full throttle or at idle and no issues, only happens when blades are engaged.

What am I missing here?!?