Picture indicates 2shanten but if I draw a 3sou or 7sou then by discarding a 6sou or ankan'ing them I should be in tenpai shouldn't I?
yes, this is 1shanten waiting for 37s: https://tenhou.net/2/?q=456p1244666689s6z
my guess is that the overlapping groups are confusing the algorithm the game is using to count shanten.