Finally, "Auto-sync data" option in phone settings causes Opera running in background. Unchecking that, issue's gone.
Phone Settings > Accounts => tap on the three dots (upper right) and uncheck Auto-sync data:
Of course, Opera Startup option should be set to "Continue where I left off".
Opera Settings > Startup => "Continue where I left off"
Setting above two options, Opera stays closed after "Force Stop" till next time I'll use it.
@sgunhouse Topic solved. Thanks for your help!