My itinerary, SAT-MEX- MAD-MAD-MEX-SAT, prices on Google Flights using the multicity option. The only booking site listed is AM.com, no OTAs, and the itinerary is bookable as separate flights. The same itinerary still doesn't appear as a round trip option.
The AM bankruptcy confirmation hearing is Jan 18 so I suppose we'll know more in the coming weeks.