In the past a Fibaro Smoke alarm test would not trigger a Homey alarm (which is silly to me, but still). This recently changed, so it could be the Homey app was not tested for this situation. I would therefore also put in a ticket at the developer (Athom itself in this case).
Tried all suggestions but the only thing that helped was firing up the test alarm by long holding the button on the sensor again and waiting until it went off and sent the reset signal to Homey.