"Export PNG Image" with non-alpha bit depth always renders black background

Steps to reproduce:

• open Inkscape
• create or load any image
• go to "File" -> "Export PNG Image" (or press Shift+Ctrl+E)
• expand the "Advanced" section
• select any "Bit depth" without an alpha channel, e.g. "RGB_8" or "Gray_8"
• click "Export"

What happened?

Inkscape renders the objects in the exported image against a black background:

What should have happened?

Inkscape should have rendered the exported image against the document background color defined under "Edit" -> "Document Properties" -> "Page" -> "Background" (which defaults to transparent white).

Inkscape Version and Operating System:

• Inkscape Version: 1.0rc1 (09960d6f, 2020-04-09)
• Operating System: Windows 10 Enterprise
• Operating System version: 1909

Sample file used for the screenshots above: drawing.svg