Stupid me. I think I know exactly what this was. Looking back at my logs I realized that I waited too long in between batches to do a straight repitch, so I made a “revival” starter to get the yeast going.
While my usual practice is to use distilled water for starters with DME, it appears that on this one I didn’t. My notes don’t mention anything about putting Campden into the starter water, so I probably just pulled it from the fridge filter.
Diluted into a 5G batch, the 2L of Chlorophenol laden starter wort was not as obvious as having the same issue with the full batch, but I’m almost certain that’s where this off flavor is coming from.