Is it true that if a person has good results with Minoxidil and stops using it, the hair will fall off again?

Yes, the hair which are present on your scalp under the effect of medication can fall off after the discontinuation of the medicine. But, you must understand that this is what the original pattern of hairfall would have been had you not been applying minoxidil. So, it is not that you start loosing again but there was a pause in the hairfall because of the minoxidil. Now, once you have removed Minoxidil, the pause has also been removed and you have started loosing hair again.  

Yes, in fact it is true that  if a person has good results with any treatment for balding and stops using it, the hair will fall off again. This applies to minoxidil, finasteride, laser, PRP