Compare your settings of the checker.
From the Accessibility panel, compare what's checked to "check" in the 4 sections listed in the drop-down menu:
ALL items should be checked, except for one in Forms/Tables: Uncheck "Tables must have a summary."
Also, are you both using the same version of Acrobat Pro? The checker utility is constantly being tweaked by Adobe.
Help / About Adobe Acrobat Pro. Current version is 2020.013.20074 (February 2021).