You may not be low on folic acid prior to taking the multi. You may have just needed more to obtain a therapeutic level. In other words, you have enough for your body to sustain life, but you needed more to heal. Folic acid is important for nerve repair, blood brain barrier and some other things. Many who have sensitivities to certain foods also have vitamin deficiencies. A gut that is angry against invading foods often does not absorb nutrients well.

So, there are two issues to consider.
Are you malnourished ? or
Are you properly nourished for a healthy body but need more nourishment for a body that is under healing stress and needs more nourishment ?

The doseages on multis is usually to prevent malnourishment.

btw. People with gluten sensitivities often are deficient in all or most of the B vitamins.