My guess is the off flavor is from higher levels of phenols from chlorine/chloramine. I had the same taste for two consecutive batches, called my water authority and found that they add chlorine to the water. I use campden tablets on every batch now and I have not tasted it since.
One campden tablet (per 5 gallon batches) ground down into a powder and split between mash and sparge water should do the trick.
Have you detected this in other batches?