Thanks @Gareth, let me know if I need to raise a ticket somewhere. I also noticed that the Navigation menu RHS was appearing in the mobile version but not in the non-mobile version of the same site. Perhaps that is also linked to the same bug?
@Scott Issue is https://gitlab.com/jezhops/moodle-theme_adaptable/-/issues/237 - I can also replicate in Boost, so might need to raise an MDL. 'Navigation' menu issue? Please explain with screen shots.