I may be incorrect and they are now using explosive bolts. I've seen those mentioned elsewhere.
However the engines were on the whole time, producing >1G of thrust along the direction of the booster. So that force is going to hold the StarShip and the booster together. But you would expect some lateral force from air friction to push the stack apart at some point.
That isn't what it looked like to me when it made what, 3 go arounds?