Found a thread from the recent past that describes the same problem that multiple users are facing and it seems that it has to be an Opera issue and not something that happens on specific devices only. In other Chromium-based Android browsers this behavior in non existent. There isn't a solution yet and it seems that developers are working on it.
https://forums.opera.com/topic/34579/opera-can-t-load-password

Found a thread from the recent past that describes the same problem that multiple users are facing and it seems that it has to be an Opera issue and not something that happens on specific devices only. In other Chromium-based Android browsers this behavior in non existent. There isn't a solution yet and it seems that developers are working on it.
https://forums.opera.com/topic/34579/opera-can-t-load-password

This is driving me crazy for about two weeks. I am trying to use Opera in all of my devices but a major issue is holding me back from doing so. I have ~90 saved logins and on my desktop all of them are autofilling but on my phone the autofill feature only works on some sites while on other sites it won't. I can see the saved login info when I search it in the saved logins menu in "Settings", I can copy and edit them but they won't autofill when I visit the website.

What's more strange is that some sites that won't autofill on the phone version have different web adress in the saved logins list than the same login entry on the desktop. For example on the dekstop the saved login for a website is this: https://www.winbank.gr/el/SiteAssets/LandingPage/index.html

while on the phone it is synced like this:
https://www.winbank.gr

I tried multiple re-installs and resync's while also resetting the sync info in my account and importing my saved logins again.
Ad-blocking and data saving on or off doesn't make any difference.

I've searched for similar answers on the web but I couldn't find something useful only a thread on Reddit that stated that Opera doesn't block the autofill restrictions of some websites. Is this true? If not what could be the solution for this issue?

Phone: Xiaomi Redmi 6A Global version
Android: 9.0
Opera for Android: 52.4. 2517.140781