The Placeholder position is not in sync with the Entry's Text, it is always top-left aligned.
Here's a quick screenshot of what happens when the RadEntry is larger than the default.
This has the side effect of not being aligned with the Text.
There are several ways I can think of to approach this, but I think the easiest way would be to just bind the top level control's VerticalTextAlignment property value on the internal Label being used for the Placeholder
<telerik:RadEntry Placeholder="Name" WidthRequest="280" HeightRequest="100" VerticalTextAlignment="End"/>