The issue where users sometimes can upload files to cloud services and sometimes cannot suggests inconsistent enforcement of Endpoint DLP policies. This can be caused by the unallowed browsers in the Microsoft 365 Endpoint DLP settings are NOT configured. Also, there are file path exclusions in the Microsoft 365 Endpoint DLP settings.
Endpoint DLP can block uploads only when using unallowed browsers. If unallowed browsers are not configured, users might be able to bypass restrictions by switching to a different browser. This could explain why uploads sometimes work and sometimes don't, depending on which browser is used.
File path exclusions allow certain files or folders to be exempt from DLP restrictions. If a specific file location is excluded, files stored there won't trigger DLP policies, leading to inconsistent behavior. This could result in some uploads being blocked while others are allowed.