When you turn it off on the app, is it grey out? And is the power light off on the smart plug?
How about if you press the power button on the smart plug?

I'd also check for any wiring issues, maybe a bad/loose ground somewhere, or, if it's an HS200, a loose neutral. It's possible but probably unlikely the relay has gone bad inside the switch, you might be able to confirm an HS200 by disconnecting and capping off the outgoing wire from it then restoring power. If the light still glows it's your wiring, if not you may need to replace the switch (it may be stuck or have some kind of power leak). The fact that you're getting some glowing means (as you know) there's still power flowing and if it is bad wiring you can get a serious shock.

I have the same problem with a HS 100. Any solution for this?

hitzi wrote

I have the same problem with a HS 100. Any solution for this?

What's the model of your plug? If it's EU plug, rotate the plug and try again.