I've had my Fenix 5 plus for a few years and with my usage pattern I normally charge it once a week.
Recently the battery life has almost halved with no change in usage, I had put this down to the battery dying due to age.
However I have now noticed that following a reboot the battery life is back to normal and will only start having issues after I have done an activity, so it seems that this may be the trigger for the drain.
Many years ago we had the Post Activity Drain issue which manifested itself in a slightly different way to this but I am wondering if recent updates have somehow introduced this issue.