Fibaro Dimmer (FGD211) not responding on physical switch

I have Fibaro Dimmer (v1) installed to a single light/switch. After installation everything works fine, physical switch (toggle, have set parameter 14 to 1) turns light on and off, on/off/dimming from the smartthings app works fine as well.

But the trouble starts after using the dimmer in a smartapp (Webcore, smart lighting), after the first instance, the physical switch won’t turn off the light anymore. In the app it looks like turning on and off, but irl the light glows happily further.

If I cut off the power of the dimmer, it will work fine again, until the next call of webcore, and everything starts over again.

Any clues on this?