Exporting to a tga doesnt support alpha maps via krita.
Actually, it does. But it sets the colors of the RGB channels to black where there is full alpha. This is the same issue I have with the png exporter also. So they may be the same bug?
Created attachment 103757 [details] example alpha export as either tga or png. Open in gimp, and youll notice the alpha channel doesnt get exported.
Link to my other bug related to a similar, but a little different issue with png: https://bugs.kde.org/show_bug.cgi?id=375873
*** This bug has been marked as a duplicate of bug 375873 ***