There needs to be an xref linking the author to the affiliation. This is missing.
<contrib contrib-type="author" corresp="yes">
<name>
<given-names>Milagros</given-names>
<surname>Ruiz</surname>
</name>
<contrib-id contrib-id-type="orcid" authenticated="true">0000-0001-7492-9873</contrib-id>
<email>m.a.ruiz@ucl.ac.uk</email>
<bio>
<p>
<bold>Milagros Ruiz</bold>
is in the Research Department of Epidemiology and Public Health, University College London, London, United Kingdom
</p>
</bio>
<xref ref-type="author-notes" rid="con1"/>
</contrib>
In this case it should be <xref ref-type="aff" rid="aff1"/> appearing after the bio element.
Affiliations
There needs to be an xref linking the author to the affiliation. This is missing.
In this case it should be
<xref ref-type="aff" rid="aff1"/>appearing after thebioelement.