Problem with a report when grouped by "Due Date"

Hoping someone might be able to help with a report I created. I attempted to group rows in a report by "Due Date," but when I do that, some dates appear as normal dates (e.g. 09/02/23) while others appear in reverse, in different format, with an odd string of characters (e.g. 2023-05-30T16:59:59Z). Any ideas what is happening? The odd dates seem to be only coming from 3 or 4 sheets spread across 23 sheets from three different workspaces. Thanks