The Garmin systems I flew were only certified for ops south of 70°N and north of 70°S. Nobody knew what would happen closer to the poles. Maybe the system was just not tested to certification standards out there, or maybe it would do something really stupid and want to fly upside down or backwards! Either way, I wasn't going to be the pilot finding out on a dark night.

It’s likely to do with satellite geometry at extreme latitudes leading to lower accuracy. Maybe also the weaker access to correction data from geostationary satellites.