If you open a multi page PDF in Illustrator and then copy and paste it either into the same document or a different one, it changes from cropping to the bounding box to whatever was specified last in the File/Place options dialogue box and scales up or down to fit the same space.

If I place a file (any file) and specify cropping to Bounding Box, and then open the same multi page PDF as before, the problem does not occur. It appears that by opening a multi page PDF it crops to bounding box but by copying and pasting it reverts to the last used cropping in File/Place and resizes to fit.

This issue is present in both Illustrator 2021 and 2022 on Mac OS 10.15.7.