


Switching between docked mode and handheld mode the controllers (Xbox game controller/8BitDo controller) were not properly detected. Relaunching the game fixes the issue.
As many people, I was not able to import the save from Mask of Deception. At the time i didn't used ProtonDB and was not aware of the trick in the console. So i ended up using my Windows PC to import the save and create a Mask of Truth save. Once steam synced the saves I was able to start the game on the Steam Deck.
Later in the game there are some screens showing the advances in the maps/story telling that i noticed a performance hit. And just to make it crystal clear, It was not during game play/battles .

TDP Limit to 5, fps to 30
Played through the whole game withouth encountering a single issue except that I couldn't load the Mask of Deception saved data, but I did what the user teefs28846 said to import the data and had no problems, I even got all achievements in my playthrough. Great battery life, plays absolutely phenomenal on the Deck, totally recommended platform on which to play the Utawarerumono series.

Flawless stable 30fps throughout
flickering alt-tab

Works perfectly fine out of the box
The game has a feature where you can import save data from Mask of Deception. But the game can't detect the save data unless you copied the save data from it to the Mask of Truth save data location
cd $HOME/.local/share/Steam/steamapps/compatdata
cp -r 1149550/pfx/drive_c/users/steamuser/Documents/AQUAPLUS/* 1151440/pfx/drive_c/users/steamuser/Documents/AQUAPLUS/
Needed to Manually copy the Mask of Deception save game to the Mask of truth Proton prefix in order to make the save game import work when starting a new game:
cd ~/.local/share/Steam/steamapps/compatdata
cp -r 1149550/pfx/drive_c/users/steamuser/Documents/AQUAPLUS/* 1151440/pfx/drive_c/users/steamuser/Documents/AQUAPLUS/
After that save game import works without issue
Unable to import the Utawarerumono:Mask of Deception clear data without workarounds
to workaround, assuming you have both games installed copy the data from the MoD prefix to the MoT prefix:
cd ~/.local/share/Steam/steamapps/compatdata/
cp -r 1149550/pfx/drive_c/users/steamuser/Documents/AQUAPLUS/* 1151440/pfx/drive_c/users/steamuser/Documents/AQUAPLUS/
then start MoT and the clear data should import without issue
Some noise appears ramdomly when playing the game. Don't know if it is related to proton, but it isn't such a botter generaly
Events with much particles causes some slowness, but it doesn't impact much in the experience
The game crashed two times while in battle. Since it saves at each turn, it's not a great problem

gamescope -w 1280 -h 720 -W 1600 -H 900 -e -U -- %command%
To note, I used a custom proton (version 7) to play the game. Also, the game's framerate is limited to 30 fps and the native in-game resolution is 1280x720. If one wishes to play fullscreen or at a higher resolution, like a 16:9 aspect ratio ratio, then one way is to install/use gamescope and enable AMD FSR for upscaling
only 1 choppy scene. that's all. it won't affect audio.
disconnect controller and reconnect it again need to switch between window mode and full screen mode to connect it again.
only one background cause performance problem. it has too many visual effect, but it didn't affect audio.
Works perfectly out of the box, has a bug with the Shisu fight, which is also present on Windows, no issues otherwise
No problems observed
Played the game to completion with no problems noted.
Game uses same engine as Mask of Deception and works great with proton 5.
There is (or was) bug that caused game to freeze if Shisu preforms counter, but it's also present when playing on Windows