Which platform do you have this issue?
I used following code, the shadow appears normally for all platforms. If you set y-offset value to negative, the shadow will move up, if you set the x-offset value to negative, the shadow will move left.
Could you share your layout code that I can make a test?
<Image Source="dotnet_bot.png"
WidthRequest="250"
HeightRequest="310">
<Image.Shadow>
<Shadow Brush="Black"
Offset="20,-20"
Radius="40"
Opacity="0.8" />
</Image.Shadow>
</Image>