Removing a shared field from one Content Type causes data lose in other Content Types using the same field.

I have two Content Types
Content Type A and Content Type B each with an instance of a node reference field. Lets call it field_nfc. I then remove field_nfc from Content Type B.

I see that the DB table content_field_nfc gets deleted and a column inside content_type_a gets added called field_nfc_nid. This field is however NULL for all rows. Meaning the data didn't get migrated correctly from the table content_field_nfc into table content_type_a.

As an aside Content Type A is used in a flexifield. Not sure if that matters. I can't think of a reason it would but I don't completely understand flexifield yet.