@RehanRamay
that is really weird. I have uninstalled and re-installed Opera over the years, and it never happened, in fact for that same reason I have 15 devices in my Sinc. data, some that are old I'm not using any more because they are mostly broken ones. (PC's, Mobiles, Tablets).
For your bookmarks to be deleted...
- Did you have "two-factor authentication" to access back to your OA?
[ TOTP (Time-based One-time Password) ]
The only thing I can think of. As data in the servers is encrypted, you can lose access to it, if you lose your old password or access to the 2-steep verification mobile/alt. email. etc
Where did you link any social media prior to your account? (Facebook, Google, etc ) !?