I'd like to get back to the original question of this post (since I'm the OP), and not the later discussion on the bitterness of FWH (as interesting as it is)...
I tapped my most recent beer a few days ago, a Schwarzbier that has been lagering for a little over 4 weeks. I've brewed this recipe every year for 5 years and never had a problem with it; it's very close to Kostritzer (I lived a few km from the Kostritzer Brauerei in Germany a few years back, biked past it a lot, and it's a top-ten beer for me). Anyways... I sample it, and there is this same weird lingering tannic bitterness and astringency that I had in the Pilsner this spring. Fairly unpleasant... with the Pils, it reduced a little after about 8 weeks in the keg but not very much.
So I have a theory: the Pilsner was the first beer I fined with Biofine Clear; prior to that i used a gelatin solution. Some quick googling shows that possibly gelatin and biofine react or precipitate different compounds differently, and that perhaps gelatin pulls tannins, astringent compounds, and hop polyphenols out of solution more effectively than biofine clear (see this thread http://www.thebrewingnetwork.com/forum/viewtopic.php?f=&t=19626
and this http://www.beeradvocate.com/community/threads/fining-agents.192243/
Of course I've brewed a bunch of other beers in between the Pils and the Schwarz and none of them exhibit this odd taste. So part B of the theory is that the excessive hop matter used to bitter these two beers is somehow involved. See, these two beers are the only two that I've bittered with large amounts (>4 oz) of low alpha (4-5%) hops for 60+ mins. Other lagers I've brewed this spring/summer were bittered with hi-alpha magnum, in <2oz amounts. (All 10-gal batches, BTW). In both beers, there was over 6oz of low alpha hops in total.
So question is, is there something in my brewing process that is creating tannins, astringency, or whatever, with large amounts of low alpha hops, and then was gelatin somehow able to clean that up but Bionfine can't clean it up?
Adding to the mystery is that these two beers were brewed with the exact same pound of Mittelfruh pellets from HopsDirect, and I haven't used those hops in any other beer. So perhaps it's just a wacky bag of hops?
Anyways, one idea is to mix up a little gelatin and dump it in the keg (it's carbed, but apparently one can still do that) and see what happens in a few days. I will try this if no one has other suggestions.
Of course I guess long term I should somehow fix my recipes or process to avoid the creating of these flavor compounds.
Sorry for going on so long,