The current DTD for the JATS writer template is for Journal Publishing (JATS-journalpublishing1.dtd), which does not permit ext-link as a valid child (https://jats.nlm.nih.gov/publishing/tag-library/1.1/element/publisher-name.html).
This update modifies the default output template to be the less restrictive JATS archiving and interchange DTD which systems like PubMed use internally to represent their articles.