Missing transparency in placed PDFs

I do have issues with Illustrator displaying transparencies of placed PDFs as solid.

Steps to reproduce:

1. Open a new document.
2. Draw two partially overlapping rectangles with different colors.
3. Set he opacity of the frontmost rectangle to 50%.
4. Save this document as a PDF with setting Illustrator Standard (see attached file).
5. Oben a second document.
6. Place the PDF file.

Expected result:

The topmost rectangle should be displayed with transparency.

Actual result:

The topmost rectangle is not transparent.

If you save the document with the placed PDF itself as PDF file, the rectangle is still transparent, so this is a display issue of Illustrator. Now it’s getting really weird: If you place this second PDF file in a new Illustrator document … all looks fine. 

I am using Illustrator 25.0.1 on MacOS 10.14.6.