I did some debugging on a testapp under wine.
It looks like alphablending a top-to-bottom dibsection results in a upsidedown image.
the alpha image is created at TWin32WidgetSet.StretchMaskBlt - CreatePremultipliedBitmap (win32api.inc:3420)
and is blended at line 3513.
the AlphaBlend routine is native wine.
I'll investigate more with some direct api calls, not using the lcl.