The N (or I, Windows is inconsistent about which letter it uses) attribute means the file shouldn't be indexed. Shouldn't make a difference here.
The L attribute means the file is a reparse point, which means accessing it can run arbitrary driver code, presumably installed by QSYNC, but who knows what it actually does, unless it's documented somewhere.
If Opus generates thumbs for JPG files but not MP4 then it's pretty weird if it comes down to this. Opus itself generates JPG thumbnails, but doesn't generate video thumbnails (that's handed off to the video components installed on your system).
You could make a Process Monitor log of what happens, which might reveal a detail of what's failing, and which components are involved.