@dodohjk Thank you for the extensive explanation.
I meanwhile "discovered" two more areas that behave the same.
I do not see any logic in why this would be like that.
I did realize we are at the edge of the range (I actually always tought it would be satellite data). But the data we see here is very accurate. When I can see rain (dark red clouds) coming it matches what Windy is showing. Just when the rain area reaches that shadow, it disappears in Windy but not in reality.
Will probably remain a mistery.
Concerning the line of sight to Sattahip: I do not know the exact radar position but I suppose it is on a 148 meter hill south of Sattahip. Mountains in the near area are not in the way. But it actually means the radar is looking (due to earth curvature) at an altitude of aproximately 3000 meter (9850 feets) over our island. That makes me wonder about how the data is collected. The dark red rainy clouds are definetly lower than 3000 meters.