In order to auto scroll with middle click on opera (and on all chromium based browsers) under linux, you must have an autoscroll extension installed.
I have been using this one for like ~8 years and I am happy with it so far
https://chrome.google.com/webstore/detail/autoscroll/occjjkgifpmdgodlplnacmkejpdionan
The reason is simple. Knowing that middle click can also paste clipboard contents, chromium (= the browser's engine) uses it for that purpose only, disabling autoscroll. And this has been the case since the very first release of chrome in linux, back in 2009.