Icon model and DWD
Comparing forecast of the ICON model with forecast in the DWD app, they seldom correlate. Are you sure, makers of windy, that ICON uses the data of DWD?
ICON is a model produced by DWD : https://www.dwd.de/EN/research/weatherforecasting/num_modelling/01_num_weather_prediction_modells/icon_description.html
Do you have a screenshot showing the differences between the app and the data shown in Windy ?
Have you checked that you compare the same ICON version? There are 2 ICON models. One is a global model with 13km resolution, the other has an European coverage with 7km resolution. Windy uses the second one.