The <gallery> tag generation has been updated to include srcset attributes for high-density displays: https://phabricator.wikimedia.org/T64709
An unfortunate consequence is that if extensions have parser test cases including a <gallery> they will need to be updated for the new HTML.
The only one I noticed on my setup with in Cite, so I submitted a patch to fix that; but keep an eye out for surprise test failures elsewhere.
It's an easy fix -- run the parser tests, find the failing test, and copy-paste the 'srcset' attribute from the generated HTML into the test case expected output.
-- brion