Changed the weight in the promote.pair.ids option in max matching. The problem was that a match X-X had less value that X-O-O. The former had weight 2 + 2=4 and the latter had 3+1=4. Now the former has 2 + 2 * 2 = 6 and the latter has 3 + 2 * 1=5 so matching prefers a 2-way matching of 2 promoted pairs than a 3-way matching of one promoted pair.