No firsthand knowledge of this issue but given that it has no gap, it would follow that it has less allowable travel. That might indicate that it requires a very tight tolerance on the seatpost diameter dimensions. Since you are now on your second frame perhaps the seatpost is undersized preventing you from getting proper clamping force.
Since it's a proprietary seatpost, you'd probably have to get dims from BMC along with allowable tolerances to verify its within spec. Hopefully your dealer can do this for you. If not, maybe try and swap seatposts with another bike at the shop just to see if you can get it tight.