Quick update: This latest keg is still drinking wonderfully. I think this is my most successful homebrew effort to date.
I cannot stress enough how important it is to eliminate oxygen pickup when brewing this beer. I've now brewed this three times, each time being more and more careful and the differences are stark.
For the first batch I siphoned into purged, open-top keg. No splashing, calm etc. Beer got worse with every pint. Ultimately crossed the good/bad line about a month in. By then it was also noticeably darker.
On the second and third batches, I dramatically changed things up. I began filling a keg with StarSan, purging what little headspace remained a few times and pushing out the StarSan with CO2. After that, I pressure transferred from the carboy to the keg's liquid out post, and connected a line from the gas in post to a bucket of StarSan.
I could not believe the difference it made! My third batch has been in keg for almost a month now and it's barely moved flavor-wise. It still has intense aromatics and that crazy, round hop flavor it had on day one.
I've kegged a lot of beer via siphon and never had it show this much difference. With this beer though, the difference has been insane!