If I save this in mb-dir/foo.el, where mb-dir is a directory containing
multi-bytes, the results (d1 and d2) of the same calls to
`directory-list' are different in the uni-byte and multi-byte buffer. It
seems that the 2 byte sequences of the UTF-8 characters are replaced by
some 4 bytes. Anyway, the resulting filename d2 names a non-existent
file.

> "The minibuffer displays" is the key point here:[...]

No, the key is that the file's existence depends on the buffer's
multi-byte status, in which the code is evaluated.

>
>> I hope that clarifies it.

-ap