Ok, I've got a spare device and I have reset it and created a brand new Apple ID and now the above code works exactly as I would expect. So this leads me to the following question. How can I reset my HealthKit for my personal Apple ID, without losing my data as I would like to keep it?

I deleted all my Health data by going to Settings > Health > Data Access & Devices then going through all the devices and apps that had written data to Health. This appears to have solved the issue at the cost of all my recorded data.

Typically, this was immediately solved by using a withAnimation block around the state change that determines the visibility of the toolbar.