If you join a sPvP match in progress, with a build that has Signets, the signet passives are not up.
At the start of a match, the Signet passives will then appear when the countdown timer starts.
It seems like somebody tried to fix the OLD bug of this by adding code to reset passives at the game start timer. So you still have to manually re-select your signets if you join an sPvP game in progress.
Tested with Ranger.