Trying to interpret what you was told by the ISP, it would mean either the openreach estimate is only 102mB, or that their own tools have never logged a sync speed higher then that, I have observed on FTTC, sync speeds only seem to get logged when a GEA test is carried out, which is why when I was syncing at 80 I deliberately ran a GEA test to get it logged. Whether g.fast is the same absolutely no idea.
Hopefully someone better can give you an answer, but I think with vectoring there is no crosstalk excuse to hide behind anymore and such a sharp drop in speed suggests either new external interference or a fault.