Function does not autoload in Fish shell

I have a fish function in a file called mvnw.fish in /Users/myUser/.config/fish/functions

The /Users/myUser/.config/fish/functions directory does appear (in first position, not that it matters) in $fish_function_path variable.

My function seems valid:

function mvn-or-mvnw
  if ( -f ./mvnw )
      echo "Executing mvnw instead of mvn"
      ./mvnw $argv
  else
      mvn $argv
  end
end

And if I source it explicitly it does work fine.
But never is it picked up automatically by fish.

What am I doing wrong?