Yes, in my environment we use pac file and the UTM is configured on the browsers via GPO. I think (but it's only thinking out loud) something block the office app to be able to open the file. It can be downloaded. It can be open in office online. But it cannot be opened via the office desktop app. The communication works between the client and sharepoint online without any issue except this one. But (and please correct me if i'm wrong) with transpaernt mode I won't have SSO (which mandatory)