My first impression here is that this is not a bug. It may not be what you expected, but AFAICS you did receive the payment you requested (and even a little more).
The blockchain is acting somewhat aggressively wrt matching margin calls. That's OK IMO, because closing undercollateralized positions is important for the health of the bitasset.
RE: BEWARE Crossing the OFFER with your BID on the BitShares DEX, or you may OVERPAY and/or BUY GREATER SIZE than you EXPECT!