I believe that the model which showed the issue was supplied by Zen, but I could be wrong.
Ah, now that is interesting for the one I own was a Zen supplied device. The original owner never actually used it and sold it as essentially new.
One of the first things I did was to update the firmware with the latest available from the ZyXEL Download Library
and then to clear the ROM-D, which is where the ISP/CP customisations are held.
It is only recently that I have been able to use your DSLstats utility (the -5.9-experimental version) and so all previous plots, the Bitloading, the SNR, the QLN and the Hlog per sub-carrier, were created with my standalone graphing utility. I have never been aware of any misalignment.
I attach both the Bitloading per sub-carrier and the SNR per sub-carrier plots, which were created earlier today.