Hi @Boris_Schaedler,
We have an active ticket regrading this behavior but I should note that this ticket is currently a feature request.
QA has noted that joining the two zones together creates a new audio group which may or may not have the same settings.
Since this is currently “working as expected”, I would suggest that you add further feedback to the feature request ticket as the product team keeps a close eye on the feature request section for where to take the product next.
Thanks!