This is essentially the root cause ticket for JENKINS-42546 / JENKINS-42599

This is caused by the workaround for JENKINS-26578 (source).

It causes the YUI button in the 'template' block used by repeatable to be rendered before the html text is captured by repeatable behaviour initialization.

Then later, when it is inserted, the behaviour doesn't work properly since the markup is not what is expected.

If I set a larger timeout (3000) in the JENKINS-26578 hack, then the new entries get added properly and the Add button works fine.