Locust Leaf Curling Issue
I realize my "post" is at least 2 months after you first asked your question; however, I just joined the forum! As I have over 30 years professional experience as a biologist/forester, I thought I would still try to answer your question in the hopes the info will still be of use to you and/or someone else.
Based on the description of most of your locusts, it appears that you have both New Mexico locust (Robinia neomexicana ) (which has "purplish" flowers) and black locust (Robinia pseudoacacia L. ) which has "whitish" flowers. However, some of your trees may also be honey locust (Gleditisia triacanthose) (which has "greenish" flowers). All three species are native to New Mexico.
From the perspective of the plant which is proving "troublesome", my guess is that the plant is either some variety which is quite "exotic" to New Mexico and/or the care of the plant is causing it to be "slow growing". In other words, although you noted that you excavated an extra large planting hole, it sounds like this plant leads a "pretty good life" when compared to the other plants. Consequently, this particular plant has no "incentive" to spread its roots, extend the height of its crown, etc. However, I may try "weaning" the plant for a couple of seasons rather than simply "cutting off" your supplemental watering and feeding "cold turkey". Good luck!