I believe you are correct... I remember Samsung Health updating the night before all hell broke loose. I can't test, because I already took the Frontier S3 back to Best Buy, but I'm betting that will fix it.
The odd thing is, when I look in the Settings for the Samsung Health app, I see the toggle for "Detect Workouts" - but it is grayed out (and set to ON) on my phone... not sure why.
OK - so this is wierd. I went into the Samsung Health App, clicked on "Connected Services" - didn't change anything, just clicked BACK, now I can toggle Detect Workouts on/off...
It definitely sounds like a software issue. I was getting 2.5 days out of the S3 Frontier on a full charge but since the last update it wont last more than a half a day. I only use the default watch faces (mainly the digital watch face in white) and have always had the auto heart rate and work out detections on and connected to my phone via Blutooth without much loss of battery life.
When I did have issues, it was usually after a software update, but I could always fix it by factory resetting my S3 and restoring it. This time has been an exception and even switching the watch to battery saving made, it have deminished at exactly the same rate as having it on normal.
Funnily enough my Samsung Galaxy S7 recently updated around the same time and is also failing to last a full day on full charge, requiring me to take a power bank with me just in case. Something definitely doesn't sound right with the recent OS updates.