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)