First off, I was informed that this topic was "quite old" when it is only a few months old. This is quite old by Opera's standards? Ha ha not funny.
Anyway, I get this exact same thing when downloading large files via Opera (lol 3 months later - thus not old). But mine happens without internet interruptions. (FYI I use a hard wired LAN exclusively) For instance: I queue up several large files for download, one disappears from the download list entirely, the others continue downloading just fine. Then, low and behold, there is indeed an "opdownload" file in my download directory representing the disappeared file.
I have now noticed this at least 3 times in the last few days. As for anti-virus, I only use the default Windows Defender (Windows 10). This is a bug with Opera - as it does not happen with Chrome, Edge or Firefox.