Hi Brett,
I also wasn’t able to view your workout and I don’t know much about your body or training history, but my understanding is that a negative value for aerobic decoupling would indicate that you are below your aerobic threshold.
Like I said before, I don’t know much about you or your training history; however, I am assuming you are middle-aged based on the threshold you selected using the MAF methodology. It is pretty well understood, and explained in “Training for the Uphill Athlete,” that the MAF method can be pretty conservative.
I would either retest at a slightly higher HR; however, you may find that you are still under your AeT, meaning you may have to retest several times until you have a good idea of what your AeT is. An alternative, would be to forget the MAF method and try to keep your HR around/below a value that coincides with your ventilatory threshold (talking/nose-breathing pace) during the drift test. This may result in an aerobic decoupling value closer to the 5% mark, and you may only have to retest once or twice more.
Shashi also proposed a pretty good alternative as well.