I have been doing the same routine for 3 months now with a stable situation all the way through FW 10. I have been using GC mobile on my WIn 10 mobile phone (Nokia Lumia Icon) with great results including an update yesterday post run. I came home and connected to my pc to check sync and any updates. Later attempted to connect to my Fenix 5X with the phone to no avail. I have done everything to remedy to no avial last night and this morning. From uninstalling GC and removing traces of Fenix from phone. Doing a reset to default on the watch. Speaking with support this morning and reinstalling a prior release on the fenix from a file he emailed me. Still no results connecting to phone. I am able to connect wired as well as sync wirelessly to my network. I simply cannot connect to my phone though it does see the Fenix and will pair with it. GC mobile also sees the Fenix but will not connect.
Note this is also posted in Fenix 5x... In any case I tested bluetooth pairing to a Garmin Drivesmart 51 and paired and made a voice command phone call between the two so bluetooth seems normal on the phone.
*Solved this doing a complete reset of the phone. GC then asked to pair the 5X.