Many ebook readers on Android look for the first html/xml/xhtml file to make the cover. So without the first file having the cover, they all look blank.
When you edit-ebook and add cover, the titlepage.xhtml file gets added. This will make the ebook readers show the cover int he app.
Fixed in branch master. The fix will be in the next release. calibre is usually released every Friday.
status fixreleased