Foals not getting out of the sack when a birth is unattended is, unfortunately, not all that uncommon with minis. I am sorry for your losses. BUT I would also not rule out the possibility of Rhino, especially since it happened twice. Rhino is better known for causing late term abortions, but the mare can carry to term and deliver a weak foal, unable to get out of the sack. Was the placenta delivered WITH the foal? From my reading, that seems to be a clue that Rhino was involved. It happened to us twice - and both times our mares were on Mare Stare and foaled with NO warning. We now vaccinate anyone who is leaving the farm every 2-3 months for Rhino/flu and make sure the pregnant mares get Pneumabort shots at 5, 7, and 9 months gestation. And of course, we are there for the birth. Testing (necropsy of foal) will not always be conclusive.
Did your mares have Pneumabort shots?