Just updated my Fenix 5X with a new GPS Update. Not sure what improvements this is intended to provide but it is available on Garmin Express, though does not show as a software download otherwise on Garmin's support page as of tonight.
There seems a GPS Chipset Type M5 software update for Fenix 3 and 5 series. Changelog for F3 3.20 update says "Fix satellite tracking issue in certain conditions." Probably same for F5 too.
It should be available when you connect it to Garmin Express. Downloaded for me last night. Bike ride after didn't show any changes but then the tracks here are usually pretty good.