The original Glicko-2 announcement (OGS has a new Glicko-2 based rating system!) said this:
“Games will no longer be annulled after both players have made a move. This is a change from our previous behavior where cancellation and annulment would take place if the game ended prior to move 19 in a 19x19 game.”
In other words, when white has played a move, the game is good and can’t be annulled. When black plays the first move, black is agreeing to the game, and therefore cannot cancel it, which is why the cancel button disappeared (I don’t know this for a fact, I am deducing it from the circumstances). Resignation under the circumstance Farraway describes used to lead to annulment, but apparently only cancellation does now. Another recent game in Farraway’s history (10840447) did result in annulment after one move, but that was because it was canceled rather than resigned. I must therefore retract my suggestion of a bug; the foregoing description seems to be how it is designed.