Filter effects not applied upon exporting to PDF with 'Rasterise filter effects' ticked

Migrated from: https://bugs.launchpad.net/inkscape/+bug/850987

Steps to reproduce:

File: Filter.svg

• Open Inkscape.
• Create two layers. Have lower layer stay in 'Normal' blend mode.
• Set blend mode for upper layer to 'Multiply'.
• Create two circles (or other shapes) on lower layer, filled red and green.
• Apply blur (e.g. of value 10) to green circle.
• Create third circle on upper layer, apply blue fill.
• Move all circles so that they overlap each other.
• Filter effect is correctly displayed in Inkscape (as in Filter_in.png): region of overlap between circles is black
• Go to File -> Save a Copy...
• Enter file name and select type PDF, click Save.
• 'Portable Document Format' dialog is opened. Make sure to tick "Rasterise filter effects" checkbox. Click OK.
• Open created PDF file with Adobe Reader to inspect.

File: Filter.svg

Filter (input)	Filter (export to pdf)

What happened?

• Blur effect (of green circle) is correctly rasterised.
• However, multiply blend effect of blue circle is not applied although rasterisation occurs, as is apparent when zooming in to its edges in the output PDF file (pixelisation is visible, easily visible if low DPI set for export).
• See Filter_out.png

What should have happened?

Multiply blend filter effect should result in the overlapping region of the circles be black (as in Filter_in.png).

Inkscape Version and Operating System:

• Inkscape Version: Tested with both current stable release 0.92.4 as well as current alpha version 1.0alpha2 (c41b861, 2019-08-14)
• Operating System: Microsoft Windows 10 Pro 64-bit
• Operating System version: 10.0.17134 Build 17134

This issue appears to apply to other or even all blend modes (only tested with a few more).