I use the equivalent of 34/70 (Wyeast liquid) weekly (sopmetimes pitching this strain several times a week) and every once in a while I will pick up diacetyl, but it is very rare. I have never, ever picked up any diacetyl from a first gen dry yeast pitch of 34/70.
One question: do you think it is possible you picked up and oxygen after fermenttaion, like from an unpurged keg or something? I had a batch of helles recently that tastes spot on on draft but will pick up diacetyl very quickly once filled in a growler. Literally less than 24 hours.
Thank you for the great info. The oxygenation is a very good thought, I will definitely go through my mental checklist and be very careful next batch. I plan on replicating this brew again with better temps and times to see if that helps as well. I also plan on taking another gravity sample in a few days, just to rule out infection, although I think it has more to do with my fermentation schedule than anything else.
As a side note I poured one of these last night off draft after drinking a commercial pilsner and I had to pour the draft glass out. The flavor stands WAY out when compared to a clean example. Unfortunately the keg will probably face the same fate when my pipeline catches up...