If you decide to update Opera or to close it via task manager (in case of unrelated to Opera lag or something) all tabs will get lost even with correct open tabs from previous session setting turned on. This also happens randomly on some crashes (most crashes do not lead to tabs getting lost).
While it would be hard to fix the reason behind this, I'd like to suggest
Restore Tabs from Previous Session or something like in Google Chrome. SessionBuddy (an extension) restores tabs in Opera One, but puts tabs from all workspaces into one, which is not really a working solution. There should be a solution specifically for Opera One and its workspaces, as it seems to be the only critical issue that I met after month? of using the browser.