Add Non-Steam-Game (.exe) fails

Hey there, I am experiencing a similar issue to this one: https://github.com/flathub/com.valvesoftware.Steam/issues/940

- Steam is installed as Flatpak on my main drive A
- None-Steam-Game install.exe is located on my secondary drive B
- I have given access to the secondary drive B and all of its folders via Flatseal
- Steam is able to see the install.exe and its folder
- I open Steam and try to install the game via said .exe, which actually works just fine
- After installation, I head over to Steam and manually select the .exe that makes the game run from its install dir on the secondary drive B
- Nothing happens, the game doesn't launch
- Tried with Proton Exp and Proton Exp community edition, same issue
- Tried with Proton 7 and Proton 7 comm ed, same issue
- Installed the game on the main drive by performing the same steps as above, same issue, game installs fine but won't launch using its run.exe
- Tried giving Steam Flatpak access to my home folder and all other file systems via Flatseal, doesn't work, Steam doesn't even launch when doing this (I assume for security reasons)
- Running openSUSE TW
- Doing this with a native Steam installation works just fine

Any ideas what could be causing this? Ty