Hi,
MSIX packages can be added to "Remote App" app group and users/groups can be assigned to the same. This should allow the separation of the MSIX apps. (Session hosts gets MSIX apps mounted from a shared locations based on the assignment). Have these steps been tried already ? https://learn.microsoft.com/en-us/azure/virtual-desktop/app-attach-azure-portal
MSIX app attach feature purpose is in fact to separate apps from session hosts.
~Ravi