And now my line is back to fastpath on the the upstream meaning I have lost about 2Mbps of speed. So this is definitely DLM making changes based on the line conditions (errors, noise, whatever).
It just seems crazy to me that I lose a whopping 2Mbps just because G.INP is disabled on fastpath. It would be far better for me to re-gain that 2 Mbps, and go back to an interleaving depth of 4 (which is so low anyway that it hardly has any noticeable impact).
It is one of those odd occasions where an improvement in performance of my line comes with a speed penalty!