@Reidap...You should provide you search query with mocked up details for us to help better. We would need to know how you are getting the multi-valued field?
For example if
UserName=User1, User2, User3
UserName="*" in your base search may give you multi-valued field when you try to gather values(UserName)
In case you have a single user selected UserName="User1" in your base search will give single user even when you perform values(UserName).
So in this case you need to Add Static default value to your dropdown for All=* then use UserName="$user$" in your search query. Drop down default value will be All or *.
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