It works perfectly for me on Fedora KDE Plasma with the latest Opera RPM.
I fixed the FFmpeg issue by replacing Opera's bundled libffmpeg with a working version compatible with the latest Opera build. After the replacement, video playback works normally again.
So for me, the latest Opera RPM works perfectly on Fedora KDE Plasma — the problem was only the bundled FFmpeg library.
I can upload the working libffmpeg and provide the link if needed.