It's very odd. It doesn't seem to affect Garmin handhelds, or my Stratux ADS-B receiver. I place my Spot and Stratux on the glareshield side-by-side, and my Garmin Aera 660 is 4" below them on the panel. Haven't seen any GPS issues on any of them with the Spot in tracking mode.
I wonder how much of this is really the Spot, and how much is the shielding design on the Garmin in-panel products. Would an InReach or SpiderTracks mounted on the glareshield do the same thing?