Hmm, that is odd. I just tested the latest version of Sela – 1.0.8 – with the latest version of WordPress – 4.3 – and previous comments are still displayed, in addition to the form on my test site.
Here are some troubleshooting steps:
1) Have you made any theme changes in your child-theme files? If so, try undoing them temporarily.
2) Does the issue persist if you activate the Sela parent theme?
3) Did you activate or update any new plugins recently? You could try temporarily deactivating all your plugins. If the problem is gone, reactivate your plugins one-by-one, testing the Subscribe button in between each, to find the conflict.