Symptoms
You try to use the New-CsAllowedDomain cmdlet to add a domain to the list of domains that are approved for federation in Microsoft Skype for Business Server 2015. To do this, you set the verification level to AlwaysUnverifiable. However, the domain is still treated as a discovered domain that has all the associated limitations.
Resolution
To fix this issue, install the August 2021 cumulative update 6.0.9319.619 for Skype for Business Server 2015, Core Components.