SSRS Expression Count distinct column with IF statment

I am trying to distinct count all values in the Incident_ID column where a number of IIF statements are met. The SSTS expression is running but the result is far from correct. When using SQL on SSMS, I am getting totally different results which makes me feel my expression is incorrect. I am also looking at the data and the data backs-up my expression isn't right.

Kindly see below. Many thanks.

=CountDistinct(IIF(Fields!Last_month.Value = 1
                AND Fields!Clinic_group.Value = "AAA"
                AND Fields!Incident_approval_Flag.Value = 1
                AND (Fields!Incident_severity_code.Value <> "BBB"
                AND NOT ISNOTHING(Fields!Incident_severity_code.Value))                                                          
                    ,Fields!Incident_ID.Value, Nothing)
                )