It’s just a RNG event when exploring the Bomb Pit as far as I’m aware.
Exploring it 30 times though without having encountered it though seems like either awful RNG luck or it may possibly have glitched out in your game.
If you want to cheat, you can always save scum it. Make a save before even choosing the Explore Bomb Pit option and then explore the bomb pit and if it doesn’t come up, reload your save file until it does.