If I cure the reason of my hair loss, say it was malnutrition, and then discontinue using Minoxidil, will the new hair fall out?

You really need to confirm the hair loss reasons with an internal medicine specialist as well as a hair restoration specialist. An internist will be able to find out the nutritional aspect of the hair loss and the hair restoration specialist can add to the diagnosis as well as come up with a plan to correct the situation.

It is better to see your photos  in order to understand the problem you have, treatment methods and/or estimate how much graft do we need to cover area . 

If your hair loss is rapid and over a period of few months than nothing to worry...you will get back you hairs after few months and you can also use minoxidil.....but in case your hair kiss is slow and progress o we the period of months to years than its most probably AGA.

The story you give suggests that other causes of hair loss could possibly be present besides just nutrition-related hair loss. Be sure to see a physician who specializes in the area of hair loss. But if the cause is solely due to nutrition and there is absolutely no evidence of androgenetic alopecia present, then yes - minoxidil might potentially be able to be stopped.

Hair loss is multi-factorial and if you know that you had hair loss from mal-nutrition and that there was no genetic component to it, it might grow back.