SUMMARY
For some context : i have 2 monitors, the primary one is an Alienware AW3423DWF and the secondary one, a 16" laptop screen in 16:10 aspect ratio and 2560X1600 in portrait mode. Primary monitor at 100% scale and secondary at 150% (cause 100% looks hella small on a 2k 16" display)

I recently tried to play Expedition 33 but performances was really bad compared to Windows. (about 30 to 40 FPS on CachyOS and 70 to 80 on Windows)
When i tried to change some graphics settings, i saw a weird resolution in borderless widescreen 5120x2160, i tried to change to fullscreen and see if my native resolution (3440x1400) was there but nothing. Even tried with RDR2 and Space Marine 2 and i have the same bug, with Portal 2, a native Linux game, it's even weirder, the game tries to combine my 2 monitors by doing 6250x2560 resolution.

I then set the secondary monitor scale to 100% and everything went back to normal but it's now unreadable on this monitor. Additional info : when seting down scale on the secondary monitor, every apps on the primary monitor got bigger and vice versa when scaling up.

STEPS TO REPRODUCE
1. Set secondary monitor that is in portrait mode to 150% scale
2. Launch any game

OBSERVED RESULT
Game set to a higher resolution than my screen that leads to bad performances.

EXPECTED RESULT
Being able to use 3440x1440 resolution.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: CachyOS
KDE Plasma Version: 6.3.5
KDE Frameworks Version: 6.14.0
Qt Version: 6.9.1

ADDITIONAL INFORMATION
PC Specs : Ryzen 7 7800X3D, Radeon RX 9070XT
Before the 16", 16:10, 2K screen, i had a 15,6", 16:9, 1080p screen and never had any issue with this one.

Here some proton logs : https://paste.gloriouseggroll.tv/?507ad681c7639572#EwqsjvWVCUP7TZYzSSY8tczgedJp28NgyeUNNzpCUH5f

And the outpout from this command : inxi -SMGxx
https://paste.gloriouseggroll.tv/?534c335904c03c58#64sE3t3LJFNNm59dsF2myFDFbUcbD1HFXxbPFhac8E3N

Thanks for reading me !