I think this is the same issue as in OneDrive Synced folders cannot detect content change and that issue looks like it is caused by something outside of Opus.
(Either a bug in the OS itself, or something interfering with it like anti-virus/anti-ransomware software, which could make sense given OneDrive folders tend to contain documents that those tools get paranoid over unknown-to-them processes accessing, as well as OneDrive triggering network activity.)
The debug logs sent in this thread don't contain any change notification events, only a few ShellChange events (which is a separate, secondary notification system that is only used for things like knowing when a filetype icon needs updating, not for general change tracking). That's similar to the other thread, but not what you'd normally see when change notifications are working. (You'd normally get a flurry of activity, just from having a web browser open.)
Change notification seems to start and stop working randomly for a few people in recent months, and when it's not working none of the change events are even reaching Opus. Some people in that thread found that running ChangeTest.exe to monitor the same drive makes Opus start receiving notifications again, which means it cannot be something Opus itself is doing; something external to Opus is blocking the notifications from being sent sometimes, and apparently allowing them through when additional notifications are set up, even ones outside the process entirely. (Which may also explain why monitoring C:\ in another tab fixes things. Maybe it doesn't need to be another process monitoring the folder, but the fact that another process doing it is sufficient tells us that the problem cannot be in Opus itself. The problem is somewhere deeper.)