Using list comprehension with 'yield' in function

[Gribouillis]

I'm clearly doing something wrong

Your generator is generating a single item which is the whole list of subdirectories.

You could return the whole generator

def scan_dir(dir):
    return (str(x) for x in dir.iterdir() if x.is_dir())

or

import os
def scan_dir(dir):
    for name in next(os.walk(dir))[1]:
        yield os.path.join(dir, name)

or

import os
def scan_dir(dir):
    return (x.path for x in os.scandir(dir) if x.is_dir())

I think the last one is better.