I had a similar problem, wherein the chain counter would count on its own, regardless of the anchor position. I had Alban in Martinique look at it and he cleaned the magnet. It worked for awhile and then quit completely. I am convinced it is the sensor that fails - not the magnet.
My approach was to replace the Lewmar A500 unit with the newer A560. It comes with a new sensor/magnet combination and uses a different technology. It requires 3 wires vs. the 2-wire system installed with the original Lewmar A500 on my A54.
Fortunately, Amel was wise and they ran a 3-wire cable from the cockpit to the windlass. Getting the old sensor and magnet out of the windlass was the biggest issue, but I was able to do that and hook up the new sensor using the previously unused wire.

The new display is a big improvement and actually self-calibrates. It was a lot more expensive ($500 USD) than simply replacing the sensor/magnet ($90 USD), but it really works and has proven very reliable. Keeping the buildup residue on the magnet is still important though.

I am a happy camper now when I use the windlass - no more counting in my head as I deploy.

Jamie Wendell
Phantom, A54 #44