New Single Page App (SPA) Layout breaks Header and Footer Customisations

Summary

The new SPA layout will ignore all Header and Footer customisation in Confluence Admin >> Sidebar, Header and Footer

Steps to reproduce

1. With SPA disabled, navigate to Confluence Admin >> Sidebar, Header and Footer, and set the Header to "test"
2. You should be able to see "test" on top of all your pages in Confluence
3. Then enable SPA

Expected results

The customisation to the header should still be visible

Actual results

The customisation disappears