last night, I joined a Heist that seemed promising, all valuables were present.
Then after playing for a while, I noticed the Host was doing nothing, not even guarding valuables and they kept getting stolen.
Nobody else was present, so that meant I was doing all the work for someone who may have been an AFK Leecher.
I tried several times to quit, but could not.
Eventually I had to stop guarding the few remaining valuables so the thief could steal them all and force the Heist to end.
According to my Alliance, this is common, someone starts a Heist and waits for other players to finish it for them.
To stop this inconvenience from occurring in the future, I am strongly recommending non-hosts be allowed to quit Heists so they can join another one that is worth playing.