@leocg
Issue solved, kind of...
https://jmp.sh/s/ME1Xs5RIIm4jqDkvWJKV
I have made a full video of the process
- Open Opera GX
 - Pop-up appears about update
 - Do not close the browser
 - Select Yes on the pop-up
 - Opera Installation unsuccessful download full Installation file
 - Select Yes
 - It installs the regular Opera browser
 
If I close Opera GX before selecting "Yes" in the update pop-up, the update proceeds as expected—Opera GX updates correctly, and the regular Opera browser is not installed. It seems the update process does not properly handle open instances of Opera GX. I did receive a warning to close the browser, but it's unclear why the process installs the regular Opera at all.