A few times I've had this happen with Fenix 5X. I thought it was cable faults (and it might be) but I just had it happen again with a fairly new cable. Now I suspect it might be updates causing it.
Last night I plugged it in, made sure it said it was charging, left it overnight and in the morning the watch had turned itself off. Turned it on and it said it only had 5% battery.
Plugged back into charger and it said it was now charging, 5%, 7% ... charge increased as you'd expect.
After charging for awhile I unplugged it and got notification of an update and did I want to do it now or later.
I seem to remember that this has happened a few times now, that it would go flat even whilst on charger and when an update has just occurred.
I certainly see other people having issues with rapid drain or flat batteries just after an update
Any thoughts?
Thanks.