If you use Vial (https://get.vial.today) to configure your keyboard on macOS and you need to map a key to backtick/tilde, mapping it to the backtick/tidle key in the Vial keyboard won’t work (you’ll get the section/plus-minus key). Instead, map it to the following code:
KC_NONUS_BSLASH
(At least that’s what worked for me on the British English Mac layout.)
@aral just curious, what keyboard do you use?