Smartthings App rules not working with iftt Webhook

Hy everybody,

i´m trying to integrate my samsung smartthings washer with iftt. Therefore i created a simulated switch that is being turned on/off by a webhook/smarthings applet from iftt. I then created a rule in the smartthings app that starts the preloaded,smartcontroled washer if the simulated switch is turned on.

The rule works fine when then the switch is being turned on via app, the washer starts.

Turning the switch on via iftt applet is working fine, the switch reacts as designed. When using the on-url the switch is turned on in the app. But is seems, as if the rule is not being triggered. The washer does not start. This happens frequently, sometimes the setup works.
Is the rule somehow tied to the app or is the rule executed in the backend as well?

What is wrong?