Consider this wikitext:

<ref name="A">A</ref>
<ref extends="A">A.1</ref>
<ref>B</ref>

<references/>

Expected result

The link to ref A is "[1]", to A.1 is "[1.1]", and to B is "[2]".

Actual result on en.wikipedia.beta.wmflabs.org

The link to ref B is "[3]". It's still displayed as #2 in the reference list, though.