AI only heal based on their overall damage level. If their damage level is below 0.5 they see no need to heal themselves.
AI with injured legs are less mobile, and can reduce the speed of the entire group to a walk. AI with injured arms or other minor injuries are less accurate. It is not uncommon for a leg or arm injury (or both) to occur without causing the total damage to go over the 0.5 healing threshold.
Ideally, AI should recognise that minor injuries should be treated, but only when the group is not engaged in combat.
A check could be made to only heal units with injured legs, arms, or with overall damage over some lower threshhold