That is not true. Most tires will loose a bit of air over time friom either the bead seat, valve core or stem seat and need a topping off to keep optimum pressures. The problem with tractor tires is that they dont look low till they get really low like 6-8 PSI.
I had a tire pop off the rim on my B26 just a couple days ago when I was working on a slope just rolling back and forth packing some dirt. The tires looked ok with very little squat to them and not low but after removing the tire and getting it aired back up with no leak found, I checked the other tires. The other rear had 10 pounds with 25 being the max so I put 20 in both rears. Checked the front -18 with 35 being max so I aired them up to 30. I think at least in my case that the tires were a little lower than needed when I got it AND combined with the temperature drop of 80 degrees it lowered even more plus I hadnt checked the air in them as they seemed to be holding up fine. The roll of the tire on the slope caused it to break the bead on the outside of the rim. It had no liquid since Kubota says with backhoe it must be removed so that wasnt a big deal other than having to remove and reinstall the tire.
Also with having liquid in 75% or more of the tire, cold weather makes liquid water shrink till just at freezing point at which time it expands. Cold weather would for sure decrease the volume thus lowering the pressure. Also possible is that the RIMGUARD installer didnt put adequate pressure in the tire to start with so it could have started with just 10 pounds or so and the cold dropped it by half at which time it came off the rim just like mine did. Luckily I didnt have expensive liquid in mine.