InDesign 2020 v 15.02, MacOS Catalina

I want to produce a book pdf with only black plate. I placed 3 grey pdf images in ID document and prepared them to use only black color (no cyan, magenta and yellow). The final result is that these images are separated to four plates in pdf book.

The solution is to save these 3 pdf images as for example tiff or psd and then place them into ID. Then the final result is right: I have a one-plate pdf book using only black color.

If this is a bug, please fix it, because it may cause problems with preparing book for print.

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Update:
I created pdf with the same image but with unchecked option "Optimize file…” and the colors are separated again. If I check this option, the problem doesn’t occur.

It’s really dangerous to have a bug like this. If I didn’t check pdf in Acrobat Pro, I would lead to extra cost of four plates instead of one.