Just found a thing.maybe can explain something. Please test if you see the same.
Here is what I have done, twice:
Reboot the phone. Check Wi-Fi access switch, it works. Turn it off.
Open Wi-Fi tether, found it works well.
When tether is working, plug in your charger. Use Samsung charger and cable to avoid unwanted discussion. The tether turned off. At the same time, there is a quick message line: what the bell, the phone take over the control. ... the spp didn't close.
I can still turn it on/off. But when it's on, the tether will automatically turn off couple of minutes. Or, you can turn screen on and off by pushing the power button. The tether app will turn off when you turn on the screen.
Not sure if this also happen on your phone. It seems something could simply trigger this non stable scenario.