Have a strange issue, hoping for help on suggestions how to troubleshoot.

I have a 3rd party tool that one-way replicates calendar items from a private calendar to a public folder calendar. Works great. However, I found 1 appointment on the public folder calendar that shows up on Mac Outlook 2011, but not in Office 2010/2013 or via OWA.

I am logging in with the same credentials from the Mac as I am from the PC and OWA (IE11). All appointments show on both systems, except for this one. It only shows on the Mac.

I have deleted and recreated the sync items, and the same scenario happens. I have trashed the Mac's cache, rebuilding the calendar from scratch, and that appointment shows. Same procedure on the PC (recreate Outlook profile), and the appointment is missing.

It MUST be on the server, because the Mac is downloading it. I just can't see it unless I am on a Mac. I can't see it on a PC as an admin, as a user with delegated access, or as the user who it originally belonged to.

Any ideas how else I can troubleshoot? Why does this only show on the Mac?