When opening a multi-page PDF in Illustrator, you have the option to import all pages as links for optimal performance. If there is bleed in the file, the size and scale of the graphic is correct UNTIL you copy/paste the link into the same or different document. Copying/pasting causes the linked graphic to resize:

1. Open multi-page PDF with bleed in Illustrator and choose to Import PDF Pages as Links
2. Select one of the linked graphics.
3. Copy and paste into the same file (paste in front makes the issue more apparent).
4. You will find the pasted graphic is now a different size/scale.

Theory: When opening a PDF file this way, the linked pages load to the "CropBox." When copying and pasting these linked pages, The pasted graphic relinks itself to the PDF using the "TrimBox" instead. As a result, the image scales the "trimmed" size up to the "cropped/bleed" size when pasting the graphic. This is not expected behavior. I had thought this was working properly in CC 2019, but I was able to reproduce this glitch in CC 2019 as well.

Workaround: Open your PDF in Acrobat Pro. Open the Set Page Boxes tool and change the TrimBox margins to match the CropBox margins (usually 0) and apply to all pages before saving the PDF. Now that the Trim and Crop sizes are identical, the glitch will be irrelevant when you open the PDF in Illustrator as links for copy/paste.

Tested using:
Illustrator CC 2020 v24.1.2
Windows 10 Home v1903