This is an incomplete answer—a placeholder for a more complete answer that I'm adding here at the request of the OP (who, I will say, has been very patient with me!).
Although the precise reasons aren't well understood, I don't think people find the observed brightening of Neptune to be especially anomalous. We have accurate photometry for Neptune for only about 60 years or so, I think, so it's quite plausible that the variation is seasonal (Neptune's orbit is 165 years). Keep in mind also that the brightening is fairly subtle by ordinary standards, maybe a couple of tenths of a magnitude (about 15 percent brighter than before 1980).
One source is Schmude et al. "The Secular and Rotational Brightness Variations of Neptune"; its first paragraph states that the brightness has increased 10 percent in "the past several decades."