Transparency disappeared after importing as a layer.

Hi, I have the following problem.
I have a PDF that has the correct images, but if I import it as a layer onto another PDF, white boxes appear around the images.

In the final editing image I have circled some images in orange to highlight the problem.
I attach files to help you understand the problem.
As you can see, I tried to insert images into the original PDF in a different way, such as PSD and PNG, (even an image already with a white box), but the result does not change.