I've been using Opera GX on another system that is on Windows 10, and was able to type Vietnamese on this browser no problemo.
I've swapped to a new device recently, and the input method (Unikey) isn't working at all, and I was only able to type English pretty much.
Vietnamese typing still works on this device, Words and other Browsers have no issue inputting Vietnamese
I'm running Opera GX on Windows 11
The Version is Lvl 7 Core 120.0.5543.85