You're not rendering a bitmap over another bitmap. You're simply copying a bitmap into a target bitmap. If the source bitmap has alpha transparency, the target bitmap will naturally get that alpha transparency too.

This might be not intuitive, but it is just logical. In a 3D level, transparent objects are sorted and blended over each other. This won't happen with panel elements. They are just drawn, one after the other, regardless of their transparency.

I don't think that it's very difficult to nevertheless produce a non transparent panel of any sort, even on a target bitmap. If you have problems, just ask and I'll help.