The scenario is the following:
Two on-prem SharePoint 2013 farms in remote locations
STS Trust established, tested and working
Farm A has Managed Metadata Service published
Farm B is connected to the Managed Metadata Service on Farm A
I am trying to map one custom user property to a managed metadata term set.
The user property is shown in the user profile and the users have permissions to add values in there.
When I go to add something in this field, I am able to add any term from the whole term store.
When I go in the User Properties in the User Profile Service Application, I can't map it to a specific term set, as the drop down is not visible (see the "Pick a Term Set for this property:" option).
In the Service Applicatoins in Farm B, I had to edit the properties of the connection to the published service of Farm A and enable this option:
Now I can choose a Term Set of all available in that term store and map the proprety to it.
The issue and solution are valid even for a local Managed Metadata Service, just edit the properties of the Managed Metadata Service proxy.