There have been several posts about this scenario, both under Windows and Linux. I can confirm that it occurs with Opera 57.0.3098.102 on both Ubuntu Xenial and Ubuntu Bionic. In my case, I suspect it has something to do with encryption of the home directory: accounts with cleartext home directories do not, in my case, exhibit the problem.

I hope this will be a clue for someone to finally diagnose and resolve this issue.