>> I found out the problem in the function "dired-insert-old-subdirs"
>> at line 1367 in dired.el.
>
> There is a similar problem in `dired-sort-R-check' and
> `dired-build-subdir-alist'.
>
> [There probably should be a function that checks for a given switch,
> and does not just use `string-match-p' in such a simple way.
> (At least it should respect case etc.)]

Below is the implementation of such a function: