Are the open joints caused by the excessive flux?  Or are both the excessive flux and the open joints caused by rework?  I would bet on the latter.  I can't visualize a scenario where the flux would cause good joints to open.  Ugly, sure, but very common with reworked BGAs.  Reworking BGAs does not always successfully make good joints on all balls, and neither does the practice of injecting flux and reheating to hope that the bad joints melt together.  It's possible and even likely that they make a contact joint vs metallurgical that fails later.  The flux is a symptom, not a cause.