QGIS 3.16+ transparency layer export to pdf not working

I’ve got problem with transparency when exporting my layout to PDF file. I have layer with transparency defined in Symbology -> Layer Rendering -> Opacity.

In map view and in layout i can see transparency in my layout:

However, adter exporting to PDF transparency is gone and edge of polygon looks weird:

My PDF export settings are:

What solve this issue, but I can’t use it because of other reasons:

• Checking "Print as raster" in Layout Settings

• Define transparency right in the color of symbol, not in "Opacity" of layer.
• Exporting as image
• using older varsion of QGIS (works in 3.2)

As I’ve mentioned, i can’t use any of this solution.

What I’ve tried:

• Enabling/Disabling "simplify geometry"
• Enabling/Disabling "Always export as vector"
• Enabling/Disabling "Export RDF metadata"
• messing up with other layer-, layout- and export settings. All things that works I’ve mentioned above.

Someone has this exact problem earlier, but no satisfying answer was provided

Do you have any ideas how to fix that issue? Am I doing something wrong?