Okay, until this gets fixed, the only workaround I've found out for KDE users is going to the KDE System Settings > Window Management > Window Rules and create a rule for Opera forcing to enable the KDE tiltebar.
(Switch to Yes from what you see on this picture)
The KDE titlebar works fine, but as a con you'll get a second titlebar up the native Opera one, as seen here:
The images are from a 2018 post about the titlebar, but the workaround still works on newer versions.
I know it's ugly to have the titlebar enabled, but at least it works, until it gets really fixed.