Custom Document Properties can create invalid files.
The following trivial code generates a corrupted file. The same works with the previous 3.0.x version. In attach you can find the file. The file had some macros and sheets but for privacy i deleted them and obtained basically a blank file that still generates a corrupted file. I believe there is some hidden configuration in the file that breaks the save with the new package version.
from openpyxl import load_workbook, __version__
wb = load_workbook("input.xlsm", keep_vba=True)
wb.save(filename = "output.xlsm")
Thank You, Lorenzo