EDIT: See Bald_Eagle's correct intepretation below..
Sorry, no advice really. The DSM is not officially documented, and I don't think us humble earthlings have much control over it, even less for those of us on the civilised side of the master socket! Though since spectrum management is dynamic, it will presumably take into account local geography and local conditions.
Maybe your DSLAM is now serving subscribers who are a very long distance from the cabinet. And to guarantee those punters any service at all, maybe the output power to nearer subscribers, like you, had to be cut back, to reduce crosstalk. Only guesswork though.
One thing that's clear - and Paul (Bald Eagle) who wrote the scripts to graph the line stats was the first to notice - the DSM of the Huawei MA5616 which is the MSAN/DSLAM in your street cabinet, is quite different to the DSM algorithm used by the ECI MSAN. So there doesn't seem to be one single algorithm in use. There must be at least two, with each being vendor specific.
We can see that your service was/is capped at 40/10Mbps. Presumably if you plumped for an 80Mbps service, you would gain some bandwidth, theoretically up to 80/20. Whether that upgrade would involve a relaxation of the DSM rules for you, no idea. But maybe an upgrade to 80/20 is worth considering, perhaps with the proviso that you can revert back if there's no worthwhile gain in bandwidth?