unexpected behavior: 'du -hs a a/b' prints summary only for a

[Parke]

Is the below correct behavior for du?

My expectation was that both du commands would print a summary for
'a/b'.  Unexpectedly, the second command only prints a summary for
'a'.

$  mkdir  -p  a/b

$  du  -hs  a/b  a
0       a/b
0       a

$  du  -hs  a  a/b
0       a

$  du  --version
du (GNU coreutils) 8.30
[snip]

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The above is on Ubuntu.

On Alpine Linux, I get the same (unexpected) behavior with both the
coreutils and busybox versions of du.  On Alpine, the coreutils
version is: "du (GNU coreutils) 8.32".

Thank you.