The details are given in https://www.reddit.com/r/nightingale/comments/1grjc7x/
In short, I found a game-placed T2 bench, and while I originally had the ability to craft weak seals (though I never did, IIRC), after adding lots of augments around the bench and then finally unlocking the ability for the T2 bench, the bench now shows the crafting options for the moderate seals but does not offer the ones for the weak seals any more.
This practically makes the game unable to progress because I cannot choose some important options going forward. I have to start over.
Gee, while making screenshots as proof of this, I found that I had indeed not unlocked the T1 crafting options for the weak seals! I had been so sure that I had them unlocked because I had seen those seals everywhere - but I obviously misinterpreted what I thought to see.
So, nevermind. This isn't a bug, it's a user error. You may close this.