Product attribute (m:m) not synchronized.

We have an attribute mainImages which is a many:many relation with mediaContainer. Before upgrading from 5.7 to 6.7, synchronizing a product from PCM also synced that attribute. Now, after upgrading to 6.7, that attribute (and only that particular attribute) is no longer synced. No errors during the sync either. If i assign the same mediaContainer to the gallery attribute on the product, the mediaContainer gets synced correctly. So the issue seems to be with syncing the m:m relation attribute on the product.

Any thoughts or advice?

TIA!