I noticed a bug in the way the id attributes are created for repeatable subform fields in the JavaScript. On line 163 in subform-repeatable.js there is this:

id = name.replace(/(\]|\[\]$)/g, '').replace(/\[/g, '_')

which in the end produces id attributes like jform_contacts_contacts3_is_primary0. Unfortunately, the id attributes in the subform templates are in the format of jform_contacts__contactsX__is_primary, so the correct result should be something like jform_contacts__contacts3__is_primary0, with double underscores.

Because the id attribute doesn't match what is in the templates, the label elements get broken because their for attribute ends up being jform_contacts__contactsX__is_primary0 or similar, that is, the X does not get replaced.

A quick and dirty fix is to replace aforementioned line with

id = name.replace(/(\]\[)/g, '__').replace(/(\[)/g, '_').replace(/(\]$)/g, '')

but I'm sure there is a smarter way, too.