Hi,
as shown in the image, in Directory Opus, all files are listed as "available on this device", contrary to what File Explorer display.
My goal is to perform a search in DO for all "sync pending" files.
In Preferences, try turning on Display extended sync attributes for cloud folders. Where it is differs by version:
Opus 12: Folders / Folder Behavior
Opus 13: File Display Columns / Options
It can also depend on the Windows versions, as Microsoft are constantly changing/breaking the API for this, depending on which process asks, or exactly how it asks.
I'd also expect Synology's software to be able to show a list of what's synching, similar to what OneDrive does via its tray icon, but haven't used it myself.