I found this on another post.
"You being signed in somewhere is controlled by a cookie for that site. So if you've set Opera to throw away new cookies on exit, that will log you out. Alternatively some third-party privacy software can be configured to delete cookies - that even includes CCleaner. So if cookies are deleted every time you restart the computer, check for something like that."
I use CC Cleaner and it is set to clear all cookies when a program is closed. I went into the cookie section, see pics, and from the left column added ALL opera items to the save cookies section. As soon as I did this I restarted Opera, CC cleaner made me do it any way to work with the cookies, and problem solved. All sync and Flow are staying connected seamlessly!
See pics