It looks like the OOUI button needs to add the class minerva-talk-add-button for this to work.
Compare:
https://en.m.wikipedia.org/wiki/Talk:Sandwich
https://en.m.wikipedia.beta.wmflabs.org/wiki/Talk:Sandwich
Developer notes
Not sure what's adding the button but it doesn't have the required class or data-event-name (see T295490)