Hi Joann, you are correct. Once that tile was discarded it may NOT be picked up by another player to redeem for that exposed joker. However if a player wanted to call it for a NEW exposure, they may do that.
I’m sorry it took so long to see this. The best place to ask any Mahj related questions is on the Facebook group “Mah Jongg, That’s It”. There are over 30,000 players active there and you can get questions answered almost immediately.