Windows implements `xinput9_1_0.dll` by dynamically loading `xinput1_4.dll` and calling into it.
In a test app linked against `xinput9_1_0`, `GetModuleHandleW(L"xinput1_4.dll")` returns non-NULL after `xinput9_1_0.XInputGetState` is called (but NULL before anything is called, indicating it calls `LoadLibrary` in the `XInput*` functions).
Steam Input depends on this behavior: it works by injecting a DLL into the game process, running `GetModuleHandleW(L"xinput1_{1-4}.dll")`, and hooking every `xinput1_*` DLL that's loaded. It notably does not look for or hook `xinput9_1_0.dll`, but it doesn't need to since the `xinput1_4` hooks affect `xinput9_1_0`.
This meant that Steam Input did not work in any games using `xinput9_1_0`, which are surprisingly common. Animal Well and Journey are two examples.