Yup a JavaScript error might well stop the layout code dead.
Have you updated staging and Theme Component to latest?
I trust you have TLP plugin uninstalled?
Oh and have you given it rights to get the required scripts?
Add this to security settings:
content security policy script src: https://unpkg.com
This is no longer necessary, because Discourse’s funky new Theme Modifiers support modifications to CSP which is now implemented in the TC
(that allows ImagesLoaded and dependencies to be downloaded)