Pedro Martínez: It's not a bug, it's Fencers choice (which is common on the other backgammon servers I play on as well):

(from the rules)

If a player is 1 point away from winning the match, he won't be given an option to offer a double because the reject would always won the match for him.

When a doubling cube value reaches the point where it would win the match for the player, the system won't show more options to double the cube value because it wouldn't make sense to increase it anymore.