Gareth, I don’t go into single view that often, but last week when I was making a video for the Moots, I switched into single view mode, and I noticed a mess. I have tried to troubleshoot this myself, even tried finding some CSS to add, but no luck. The problem is illustrated in my graphic below. The first graphic shows how it appears in a one-section view. The second graphic shows the regular Moodle way of showing the topic/topic navigation, as a suggestion to you. Maybe the Moodle way could be moved below the Topic text?
I am using Boost, and Moodle 3.9 (Build: 20200615).