I have the same issue (with ROCK on Intel NUC and Roon Remote on Windows 10 x64). This is since I installed the last versions: Roon 1.7 (build 571) + ROCK 1.0 (build 219).
It is happening a few times a day. Music is playing via network, but Roon Core is not available.
There is something new: I leave Roon Remote on. There was no music playing, and no activity on Roon. After a few minutes, I came back, and Roon Remote was disconnected from the Roon Core.
This is happening on two Roon Remotes on Windows 10 x64, on two completely different computers. There wasn’t any change on my environment (network, audio devices, computers etc.) from long time ago.
Something is massed up inside last version…
Are you using the latest version of QNAP? There have been some changes that might help with this, but it is still going through the QNAP App Store cycle, you can install a manual copy in the meantime though, see: