I have (finally) been able to replicate it
For whatever reason, it cannot happen on a first attempt, it has to be after using the 'R to restart' thing, which is probably why I've been missing it for so long
Viewing post in DELTARUNE - Alternate Ending - Chapter 7A Kris VS Susie comments
And now that I know that, I can locate the issue easie...r...
I see the problem
I forgot to put the 'check for death' thing in a loop upon subsequent attempts.
And the way 'check for death' works is:
when Susie's HP is less than 1:
Check if 'HOPE' is active
If yes, set HP to 1
If no, die
but on subsequent attempts, this was not in a loop, so after it triggered once, (and hope was active, so it didn't kill you), it just... stopped
So it turns out I'm just stupid I think