Yes that is what I thought, either 0.00 or full speed, but maybe sarabande explanation goes in the right direction: I compared SOG and log readings while approaching a pontoon exactly parallel to the current, perfect ferrygliding

I had SOG going little by little to zero, say 0.2-0.3kt while approaching the pontoon, while speed on water was at 2-2.5kt.
This until I was a few metres away from the pontoon.

Then I had to moor the boat and could not read the instruments any more... you guess it, once mooring ropes had been made fast, I went back to the displays and they both showed 0.0 (sog and over water)

Incidentally, I tried to measure things a bit more carefully with my boat, but with at least 3-4 other boats I chartered in places like the Sound of Chausey, Little Russel channel, or gulf of Morbihan no one of them actually ever indicated any current while stopped