PDF transparency is lost from Gimp2.8 to 2.10

GIMP version:2.10.22

Note: bug reporters are expected to have verified the bug still exists either in the last stable version of GIMP or on updated development code (master branch).

Operating System: Windows7 and 10

Package: installer

Description of the bug

For digital signing of PDF documents I exported with Gimp2.8 a transparent image to PDF. The PDF has also the transparency. With Gimp2.10. the transparency is lost. The image in the PDF has a white background.

Background information: Acrobat Reader offers to create a digital certificate or you have a purchased verified certificate. With this you can sign digitally the PDF document. If you create your own certificate, the other party must classify it as trustworthy the first time. Afterwards it can be recognized when the signing (not PC time) took place and whether the document was changed after signing. Appearances with graphics are possible for this digital signature. However, only PDFs files are possible to use in the appearance of the digital signature.

I will attach a transparent PDF file created with Gimp2.8. In Acrobat Reader you can see the transparency if you click on: Edit - Settings - Page display - Checkbox "Show transparency grid".US.zip

Reproduction

Is the bug reproducible? Always

Reproduction steps:

1. transparent imag
2. export to PDF
3. check the PDF in the Acrobat Reader with preference "show transparency grid"

…

Expected result: transparent background

Actual result: white background

Additional information

If you have a backtrace for a crash or a warning, paste it here.