I get the feeling the quests are a bit touchy after all the changes with Tempest. The vaulter's still seem to have trouble with the winter shelter from what I watched on a stream and now, in my own game Forgotten step 6 just... forgot to fire.

I took my sweet time finishing Quest 5 (Research 30(?) Era 3 or below techs, iirc). Once it completed, something felt off because no new quest window popped up. I thought perhaps it had been delayed somehow, so I've had my fingers crossed it would fire because... reasons. Internal checks, some random thing might set it off again. Unfortunately a few saves and about 50-100 turns later on the longest time setting and still nothing.

Short version: The main Forgotten Quest no. 6 just didn't launch. 

Arf.... ><

So, I just tested on my side and I succeeded to trigger the chapter 6:

Do you have a save file just before completing the chapter 5 quest?

 Thanks again, cheers~!

Mel