Milestone Status Limitation
|Knowledge Article Number||000171150|
|Description||Why can't I use filter logic when Milestone Status is on the report Filter?|
|Resolution||Milestone Status field on Case is a special field where the value is dynamically populated using methods different to most other standard/custom fields.
It is not available/cannot be used in many areas of an org, such as formula(reference), Custom report type, workflow, filter logic.
Have a picklist field (e.g. Status or a custom picklist field) that can be populated dynamically basing on the Milestone Status Value. After having the picklist field, create a formula field that will set the value same as the Milestone Status basing on the Picklist value that has been set.
If your case status = abc and this is the criteria for milestone a, then you code your formula field to return a value of milestone a if the status = abc.