Big Picture Mode Keyboard inconsistency

Hello,

While playing a game through BPM I noticed an inconsistency between how the keyboard behaves on Windows/Linux and how it behaved with Proton.

On my machine, both on Windows and Linux, I have the Romanian keyboard layout set as the only layout used by the OS. It is basically the same as the US International one with the addition that using AltGr (right Alt) in combination with certain letters produces diacritics (e.g. AltGr + s = ș), or certain symbols like € (AltGr + E) and others.
If I use BPM to play native games, and I use the physical keyboard to write the text while the virtual keyboard menu is active (because I don't always want to type using the SC), AltGr works as intended. When playing a game through Proton nothing happens. It's as if the Romanian layout is not in use.

I can confirm that if I manually set the BPM Keyboard layout itself through the BPM Settings menu to Romanian, and I select the diacritic from the virtual keyboard UI, I can see the diacritics in question in the virtual keyboard UI, and I can also use them - so it's confirmed that they are not blocked. Submitting the text with those diacritics also works and displays fine in the game.
The only issue is inputting them through the physical keyboard (using AltGr).

The issue names no source file, test, or entry point. Start by reproducing the AltGr input path in Big Picture Mode with a Romanian layout for a native game and a Proton game, then trace the physical-keyboard handling. Done means AltGr-generated diacritics and symbols enter Proton games consistently while virtual-keyboard input remains working.