I’ve had this happen more than once and not just on the front jacks, sometimes the rear. What I’ve noticed is this can happen after the jacks have been in a bind. Perhaps trying to level on a very unlevel site and putting side stress on the jacks. Try operating the jacks fully. Full in and full out and try not to put a side load on them if possible. I’ve also noticed that wiping some transmission oil on the jack shafts helps too. I’m guessing the seal can get slightly displaced or maybe even get some dirt in between the seal and shaft which can cause a slow leak.