It's a known, long-standing bug. Don't think there are any workarounds. I've never been able to reproduce it myself, but from other users, I guess it only happens after Opera has been closed for a while. You can close Opera and open it back up and it won't happen. Seems to happen to users more when they shut down their computers when they're not using them I guess.
You can goto the URL opera://about, take note of the "profile" path, close Opera and delete the "Sessions" folder in the profile folder. This will wipe out all tabs in all workspaces in all windows and all windows themselves, but it'll make Opera start with fresh session files to ensure there's no corruption.
Then you can see if the issue happens less or not.
One thing you can do though is to make sure you fully exit Opera yourself before shutting down or restarting your computer. Don't leave Opera open and let the operating system tell Opera to shut down. You can also try fully exiting Opera and give it a few seconds before putting your computer to sleep to test if that helps.
If on Windows, you can disable disable fast startup and restart Windows to see if it helps any. You can also adjust your power plan to make sure your computer only goes to sleep and never hibernates to test if it helps.
But, it's likely none of the above will help.
If you use Opera Sync, you can try disabling Sync for a while to test if the issue goes away.
You can close Opera, copy the "Sessions" folder to a safe place and if Opera starts up with your tabs messed up, close it, delete everything in the "Sessions" folder and copy the files from the backed-up "Sessions" folder to the now-empty "Sessions" folder in your profile folder and start Opera to test if it helps. That will get annoying doing that all the time though.
You can make sure the time, date, time zone and daylight savings settings are always correct on your system so Opera doesn't get confused with the timestamps and the session files. Probably won't help either.
