Today I searched for a rather simple itinerary: NYC to VIE, one way, Economy, 1 passenger Dec 21st +/-2 days.

Cheapest according to Matrix: AB at $616 (via DUS) (and many other option for various price points, from direct to 2 stops)

Then I did the same search adding "X" as an option.

Cheapest according to Matrix: same as above, AB at $616 (via DUS)

Then I did the same search but with "X:SVO" as an option.

Suddenly a cheaper flight with SU for $ 547 was found. The travel time is not extreme in any way and very comparable to many othe results show before.

Why does the Matrix miss the cheapest result? Bug or feature?

Even after repeating the original search, the cheaper SU connection was missing from the results. It doesn't seem to be a learning system. Not a single SU flight was offered.