Sorry, I forgot you said you had set your controller to connect in keyboard mode. Is there a reason you dont want it acting as a joypad?
To remove confusion I’d get rid of the joypad controller file, then you know its not conflicting (although I’d use the joypad in a joypad mode really).
Does it still change the shader when you press select on its own?
Can you attach or put on pastebin your updated retroarch.cfg?
Are these lines in it?
input_shader_next = nul
input_shader_prev = nul
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