This is a fact. I posted something yesterday as I was going nuts with my new BT Infinity. I couldn't imagine why when I was using the line (DL) the number of errors would start to peak up, and the line would become unstable.
I did some testing yesterday after reading a paper work (study case), that PLC and VDSL2 share the same frequencies. Turns out it's spot on. If your homeplugs (PLCs) are less than 1m far from your DSL modem/plate then the noise (which affects the same 0.2-17MHz range) will affect your speed and SNRM.
I have moved the PLCs through a cable to the other side of the room, while keeping it 1Gbps speed all through and now, no noise....line is up from 50M synch to 62M synch which is the max my line can achieve. I just need a bit of patience to get the DLM to stop and put G.Inp back in place and it will all be good.
Definitely this should be informed by the HomePlug vendors as it can create so many issues without even noticing where this came from.