I have an itinerary that pushes a lot of the boundaries here and I've explored it a bit.
The computer will not allow 2 transits and 1 stop in the same city.
I had tried to transit twice in Sydney and Stop in Sydney as well. (current Itinerary doesnt do this)
I got pushback from one HBA saying I can't stop at Aus twice which I tried to counter by saying I have a dummy booking open right now with a valid MEL and SYD stop. HUCA and the second HBA agent suggests that right now computer might not like the 5 stopover with 2 surface sectors (but 4 stopovers with 2 surface was ok and ticketed previously).
My last OWA was a 4 stop, 2 surface with 2 transits & one stopover in DOH back when that was a live option.
There are lots of contributors that have a far better knowledge of OWA so hopefully someone will be able to help or provide a reason.