2015 Suzuki TAC fork airpressure problem

Hi. I read hear some topics about Showa TAC forks, but did not found information about inner and balance chamber air pressure problems. So, I have 2015 Suzuki rmz450 and the problem is, that the air pressure in inner and balance chambers rising when i´m riding. After 20 min ride air pressures rise about 3psi in inner and 5psi in balance chamber. I think that the problem is not air temperature rising, because now when I´m using nitrogen the problem has nod disappeared. Any ideas why is that so? Forks getting during the ride stiffer and stiffer!

Would be grateful for some help on this