They starting only after additional players joined to queue.
May be some issue with a counter variable which test queue before flushing queue to a game ?
Below image is from beta site

Similar issues were seen and discussed/mentioned during following past events on live site too.
December - 2021 - Monthly challenge
viewtopic.php?f=1&t=237113
November - 2020 - Monthly challenge - Master key games
viewtopic.php?f=786&t=234755