My fenix 6X began with sudden battery drain since september.
I have written this here in another topic.
I've tried everything, from soft reset, hard reset, unpair it to garmin express and connect, remove non garmin apps, turning off wi-fi, bluetooth, etc.
Since september, i have 2 days battery without activities.
Yesterday, i saw a possible solution written here in the beta version 16 post:
Install beta version, charge the watchto 100% and then do a factory reset.
I did that, and the watch began the configuration process (age, height, wrist, etc).
That it sudden turned off and began a endless boot loop between the blue triangle and the name "garmin".
It did that for 3 hours, until it runed out of battery.
I've charged it and tried pressing the light button for 20 seconds, and press the two right buttons until hear a beep and then release the first and after a second beep release the other.
My fenix 3 sapphire had problems with bluetooth, my fenix 5x had connection problems, and my fenix 6X has a battery similar to an Apple Watch and doesn't start.
I think this will be my last garmin