this just happened once, i only have the one playthrough. but is this fixable on my main save id rather keep it for future updates
I don't know, since I'm not sure what the thing that causes the problem is. If you can live without that one quest, I think there shouldn't be anything else that could break the save.
I asked permission to download the save file. I can try resetting the quest, but other than that, I don't know how to help.
https://mega.nz/file/DaRRER7L#0Tx75At_YSvnNcHbbt8xRS6cz6MAJFJcOsN7fBpCmYM
This should work. Not sure if it'll break again in 3 tries, but it works now at least.