Nurse mare foals can’t be kept with their moms if the mom gets 'called up,' because there’s not a guarantee of enough milk to go around, and the more valuable foal is prioritized.
Horses that are of draft stock are naturally heartier, possess strong maternal instincts, have a lower breed track record of rejecting foals and, most importantly, provide the most milk.
Your orphaned foal should also be raised around as many other horses as possible so they can develop the necessary social skills and basic ‘how to act like a horse’ skills.