It isn't Mercaptan is it. or the 3 methyl 2 butene thiol?a sulfuric, putrid, skunky aroma, typically due to a reaction between hops’ alpha acids and sulfur compounds in the presence of sunlight or electric light.
Or is it moldy, musty, and cheesy from oxidation? Either one is from Alpha acids and Sulphur reacting to light or oxygen. Its not sitting in the light is it, transferring from keg to keg outside? Or fermented with the light left on?
It has seen very little light as it was fermented in a chest freezer then straight to keg.
What I am experiencing is not noticeable in the aroma and it has not made the beer undrinkable. I seem to be more sensitive to it and my buddy commented that is how all pilsners taste to him. It could definitely be oxidized however it seems to be showing signs of it earlier than I would expect. As mentioned earlier, the first beer seemed to improve not worsen however it took a lot of time and never really got 'good'.