Glad it was a comms issue rather than lack of understanding. I haven't used any Magellan products.

My Garmins haven't given me any surprises like this but it seems possible that a non-optimum satellite constellation combined with slowwalkng speed could confuse the algorithm that does the calculation.
Or the unit is broken in some way.

Compasses are cheap and reliable. This is good warning to always check the tech gadget against the stone-simple gadget.

unimogbert