The stoppage description is consistent with the slide travel being retarded in some manner. That could be insufficient recoil impulse or the shooter interfering with the slide. Since the same shooter had no issues with slightly hotter ammo, Occam's Razor suggests insufficient recoil impulse. Hotter ammo until the pistol is nice and smooth is what is needed. I would avoid "hyper-velocity" ammo and stick with 38-grain or 40-grain copper washed high velocity ammo.
A brick of Aguila 40-grain HV is a very economical load. It is made in the old Remington plant in Mexico now run by Tecnos Industrias. https://www.targetsportsusa.com/agui...-p-110823.aspx