Interacted with the table thing before talking to Lauren. It says there's a stabilised heart there, but talking to Lauren just shows the "You're back" dialogue. 

Alright, I downloaded the game and when I talked to Lauren first and then interacted with the table I got the same result. No gun eye thing.

Playing the web version and I got the same problem.

I ran into the same problem on both the Mac and the web version

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.