The bug

The /function command with if or unless always fails when it is executed in a function.

How to reproduce

1. Create a function custom:test

   function custom:test2 if @s
   

2. Create a function custom:test2

   say test2
   

3. Run the function custom:test

   /function custom:test
   

   → "test2" is not printed

Note: As workaround you can use /execute

execute @s ~ ~ ~ /function custom:test2 if @s