I can't use nested subfolders

I'm trying to use a subfolder within a subfolder for my functions and the game doesn't recognise the second subfolder.
Example:
  -functions
    -function_a
      -function_a_i
        -function_a_i_1.mcfunction
        -function_a_i_2.mcfunction
      -function_a_ii.mcfunction
    -function_b.mcfunction

The game will detect function_b, the function_a folder and function_a_ii, but not the function_a_i folder and subsequent functions or folders.