Exclude subdirectory not working with grep command

This works for me, in that it excludes the "foo" directory from the root directory of the search:

grep -rn --exclude-dir=foo 'custom' 

However this doesn't work:

grep -rn --exclude-dir=foo/bar 'custom' 

But the "foo/bar" directory is still searched. I also tried it with quotes:

grep -rn --exclude-dir='foo/bar' 'custom' 

I'm using Ubuntu 20.

Update

Although not perfect, I used this workaround instead:

grep -rn 'custom'|grep -v 'foo/bar' 

This will fail to find lines that contain both "foo/bar" and "custom".

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