Did you guys do the last security update on your phones? Me and my wife have the same s8+, i got the oct update she did not, if i conect my gear s3 to her phone the battery of my gear s3 is fine, if i connect it to my phone the battery drains in like 7h from 100 to 15%.
I have been a Samsung customer for years, and ok, this may be a software issue. Microsoft had an issue with some of their Surface Pros. It was relegated to one of the two brand of battery that was used. They at least ACKNOWLEDGED there was an issue, that a fix was being worked on, and regularly updated their customers. That's more than Samsung is doing. IF only Samsung support would ACKNOWLEDGE that there is an issue, and they are working on it, it might save some face for them, and help to retain some customers.
Today, I have packaged my Gear Sport that I purchased in May, and will be returning it to BestBuy (I bought the GeekSquad protection plan for it).