Many thanks for the video! That shows the issue very clearly.
It looks like change notifications (the place where there's usually a problem if there is one) are working OK, but when Opus tries to query the directory or details of the new items it is failing somewhere. That's quite unusual.
That a find on the folder is missing the files is very strange, as that would not depend on change notifications at all. It's like the files are (sometimes) being hidden from Opus by the operating system (or some involved component like antivirus).
Would it be possible to make a Process Monitor log of what happens when a new folder and a new text file are created, as in the video? That should tell us if Opus is responding to the changes by trying to check if the new items exist and get their details, and may reveal which operation is failing and which components are in the callstack when it fails.
Regarding Windows Defender, there's also an anti-ransomware side to it in addition to the realtime scanning. They seem to be separate things, so one may still be active while the other isn't. That kind of thing is most likely to be involved if the OS is hiding details of files from (some) applications, although it may (of course) also turn out to be something else.