I think it’s a red herring, meaning something else is or was going on that neither we nor the OP have detected. Which is why I asked the OP to see if he can recreate the phenomenon by restoring the original conditions. It cannot be the platter per se because we see the pumping disappears when LP and mat are removed. Funk Firm mat contains no metal either. I’d bet the pumping has either directly to do with balanced mode or has nothing to do with what we see.
I certainly don’t blame the OP for not wanting to change back to XLRs just for my/our benefit.