Reporting for Opera beta 59.0.2904.53502. The problem is not yet fully fixed:
The YouTube thing was fixed, this is 0K. In AliExpress it also works right or so it seems. The only quirk I noticed is the user agent switch in the three dots menu can be a little bit late to update.
But as you can see in the video, Opera is still incorrectly defaulting to mobile user agent or failing to inherit the user agent of the calling on window.
You may already know (or not), but let me say Chrome reads and uses the user agent which finds in the /data/local/chrome-command-line file (permissions: rw- r-- r--). I am only saying this because you (Opera developers) seem to be having difficulties with something which should be as easy as eating ice cream, which at least tells me the underlying engine was never designed (?) to use multiple user agents.
Cheers for now!