Sharepoint Hubsite in Teams with Viva Connections - hub menu bar disappears in SP document libraries

Hi. I have added my SharePoint Hubsite to the MS Teams main menu, as an App, using Viva Connections. This allows my users to access their SharePoint sites (where we keep most of our docs) direct from within Teams. The Hubsite tool bar contains links to all my team sites and their document libraries. Like so....

I've noticed that when I open a linked site's library from the Hubsite menu bar above, the hubsite navigation (sometimes) disappears in the library screen, so that my users get marooned in a library. 

When this happens the Hubsite button in the Teams Navigation bar also becomes inactive. And the Teams back arrow doesn't work either. Is this a bug? Any suggestions would be gratefully received.
Thx