I now remember where I first saw a reference to not filling the front tires with LIQUID ballast, don't know about foam, but when I was editing a Kioti owner's manual, there is a !caution box that states, "Do not fill the front wheel with liquid to maintain steering control."
I remember that after I read that I was able to find some articles that backed that up, but so far, I have not been able to rediscover them, but I'll try, as time permits to find them.
I'm not sure how steering control would be compromised, but I would surmise that Kioti's engineers must know something about it or they wouldn't have included this in the information I had to work with.
The only thing I can think is that the tires should normally be filled with the valve stem at 12:00 which will result in a 75% fill meaning that the water will be swirling around while the tractor is moving, and turning could possibly be compromised by the action of the water continuing to move while turning.
I could see that this could also cause some extra wear in the mechanical 4WD linkage, but I'm no expert, just my thoughts.
I must correct my earlier statement about lead / lag, as that is an issue with putting the wrong size front tires on a 4WD tractor as the rears and fronts are geared to work together with certain size tires, not anything to do with liquid ballast.
I hope we can research this a little more and find a definitive answer, it's interesting, and may save some damage to someone's tractor, or relieve remove any reason not to add liquid ballast to front 4WD tires.
John