Then try installing sdl2 again like before.
Thanks @Shahroz. I got it working. The Steam Deck has a read only mode by default for the system files. https://help.steampowered.com/en/faqs/view/671A-4453-E8D2-323C and they warn installing via pacman may be overwritten in future Steam OS updates.
But running the Windows version via Proton works beautifully. Appreciate the help.