My Controller Tests Perfectly Here But The Game Still Will Not See It
You are not going mad. The browser and Windows both read the controller directly, but most games read it through Steam Input or XInput, and…

Linux And Input Subsystem Engineer at Gamepad Tester
Checks that a controller reads the same on Linux and Steam Deck as it does on Windows.
Lucas works on the Linux input subsystem. He follows how evdev registers a controller, how Steam Input sits on top of it and where the two disagree with the browser's view of the same device, which is the usual reason a pad behaves differently on a Steam Deck or a Linux desktop than on Windows. His testing targets parity: the same controller should produce the same reading on every operating system, and where it does not, the difference should be documented.
On Gamepad Tester he handles the Linux and Steam Deck questions.
You are not going mad. The browser and Windows both read the controller directly, but most games read it through Steam Input or XInput, and…
Each answer is written by the specialist who owns that part of the hardware, tested on a real controller, and checked by a second team member. How we verify answers