I know '6 feet' quest is tied with 'echoes of yesterday' quest, and I managed to mess them together.

I took up '6 feet under' quest, completed it but didn't report back to sigfrid because it was night and I couldn't find him.
I temporarily forgot about it and took up 'echoes of yesterday' quest, and completed it.
I then proceeded with 'gold rush' which follows it and completed it too (siding with the order).
now sigfrid doesn't have any dialog concerning '6 feet under' and I'm left with the quest hanging.

Yep, that's how it happens. There's nothing more you can do about that, short of reloading chapter 3 and starting again  :peace: