SharePoint 2019: Modern pages not showing mobile navigation

We have a problem on SharePoint 2019 with the responsive mobile view not showing navigation on modern pages. The suite bar just renders as empty.

We're on the latest (October 2019) update, and I can't see anything else obviously wrong with the site. The issue happens on multiple site collections.

You can see the navigation disappearing in Chrome's mobile tools by shrinking the window width

Has anyone else encountered this?