I got my MFU by calling.... as its an open jaw... LCY-JFK//BOS-LHR.
At the start of the call I explained I wanted an open jaw, and told the reservations person I was surprised to find LCY availability online on the WED... they agreed it was surprising but booked it very quickly.
Both my MFUs have been open jaws and booked by phone and I've always had great service.
Maybe the rules have changed since?