In SiaB, downstream DHCP sometimes trapped at OLT instead of agg switch

In SiaB, we see downstream DHCP packets get trapped at the OvS agg switch as expected. However sometimes we also see them trapped at the OLT NNI port. This suggests that the latter packets are not trapped at the agg switch and continue to follow the downstream data path until they reach the OLT. It's not clear why this would be happening.