I think I figured out what went wrong on my end.
After fighting the lady of the lake and getting the heart, as I was making my way back to Deep Town, I remember falling into the water a couple of times. Then when I got back to Lauren’s house, my inventory was wonky. I didn’t have the heart with me anymore and some of my basic items like autonav, shrine hack, even wings were gone (but I could still dash).
I think this also happened before this, because when I first found Lauren’s house and tried interacting with the shrine beside it, it threw an error. Maybe because my shrine hack disappeared (though I didn’t check at the time).
So this time, I restarted and fought the lady of the lake again, then I went back to Lauren’s house, making sure I was unscathed. The black heart didn’t disappear from my inventory and I got to place it in Lauren’s heart chamber. Everything’s working now!
Later, I’ll try to reproduce the bug where things started disappearing from my inventory.