I'm having the same issue on my 42mm bluetooth model. The battery drain seems a bit better after a factory reset, but it won't measure heart rate and doesn't seem to be registering steps.
Yes! I thought I was going crazy, But in the last week my 46mm went from working fantastic, to the battery being completely drained in a matter of 4-5 hours TOPS, It also went from taking 2hrs to charge to over 4!
If this is due to an update, they need to fix it ASAP as I use this watch daily at work, and I used to average 15+hrs of use.
edit: This is an edit to add, I only get 15+ hrs of use with my watch as I keep the brightness high, and display always on as well as many other features that drain the battery, without those it would go 2-3 days.
edit #2: Now 2hrs since taking it off the charger, Down to 66% battery, this is ungodly wrong. Fix this samsung!