I found the problem. It was in the site’s CSS:
#ember19 {
display: none;
}
If you have code like this Discourse warns in the CSS editor:
Sorry – using
#ember
or.ember-view
CSS selectors is not permitted, because these names are dynamically generated at runtime and will change over time, eventually resulting in broken CSS. Try a different selector.
Presumably this was used to turn off some element of Discourse in the past, but the id was assigned to a new element after the upgrade. Whatever the case, removing this code fixed the problem.