Appreciated

but eventually I got it to work. I'm not 100% sure what it was, since I restarted the game and reloaded the save before trying as well, but in the end at some point it simply progressed as intended.
First attempt after game restart, I went talk to Zero > Cutscene ("Leah spun around") > talk to Alice > Cutscene ("Leah spun around for the third time"), after which I was sent back to the beginning before I had a chance to talk to Hazel (Narrator: "This isn't how it was supposed to end...", "Try again! (rewinding)").
After the rewind I first talked to Zero, but that time it didn't trigger the first cutscene. Instead I could immediately talk to Alice. Then it triggered the cutscene ("Leah spun around"), and I could talk to Hazel and got the proper dialogue with kiss and sneaking the collar into her pocket. Afterwards, cutscene ("Leah spun around for the third time") and it ended.
It seems like the order of events is the issue, with a cutscene triggering too early, sometimes.
The above sequence of events is reproducible, at least with my save. I could still send it to you if you want to test it - what's the save path?