linear gradient background which has stops with different alpha component is saved to pdf (qpdf.cpp) as SMask with /S /Alpha which is not interpreted correctly by acrobat, instead it renders the black box.

I found that changing to /S to /Luminosity and switching transparency Group's color space to DeviceGray fixes the issue
{{
19 0 obj
<<
/SMask <<
-/S /Alpha
+/S /Luminosity
/G 18 0 R
>>
>>
@@ -225,6 +225,7 @@
/BBox [ 0 0 1280 600 ]
/Group <<
/S /Transparency
+/CS /DeviceGray
>>
/Resources <<
/Shading <<
}}

probably duplicates QTBUG-36301 "Export painting in pdf or svg file, can't draw line with lineargradient pen."

(edited related issue)