I played a game where I played Disciple on black market. I gained a card off black market, but then the second BM never got played. I properly gained a second copy of Black Market, but the game just continued.
My guess is you didn't have enough money to buy anything from the black market the second (or the first) time.
I'm not going to defend this interface at all (it's really bad you don't see anything happen) but it's super unlikely the 2nd black market play didn't happen. Next time, if you add a game number to the bug report, we can actually check it.
I think this was from game: 2796510 Turn 10
If it was that game then it is just unclear, no rules bug.
After first play, only $2 no treasure in hand, all 3 cards cost more than $2.
So it doesn't stay on screen long enough.
Turn 10 - Rabid
R plays a Throne Room.
R plays a Baker.
R draws a Disciple.
R plays a Baker again.
R draws a Tactician.
R plays a Disciple.
R plays a Black Market.
R reveals a Bandit, a Silk Road and a Counterfeit.
R puts a Bandit, a Silk Road and a Counterfeit on the bottom of the Black Market Deck.
R plays a Black Market again.
R reveals a Sea Hag, a Wild Hunt and an Avanto.
R gains a Sea Hag.
R puts a Wild Hunt and an Avanto on the bottom of the Black Market Deck.
R gains a Black Market.