56.0.3051.104 on 10.13.6/High Sierra and it's still happening.
I've been back on Opera for about a month after many years absence, and this one bug is becoming a bit of a show stopper to an otherwise excellent piece of software.
As others have said, Cmd + Shift + T to reopen tabs, or once opened, close Opera completely/properly (Cmd + Q), then reopen and that seems to hold it off for a while.