You are trying to filter "Picklist" entity with an MDF Picklist Id.
PositionMatrixRelationshipType is an MDF Picklist Id.
Query: https://api8.successfactors.com/odata/v2/Picklist?$filter=picklistId eq 'PositionMatrixRelationshipType'&$format=json
Issue : Query is not returning any data.
The "Picklist" entity only retrieves picklist from PICKLIST table, which can be exported as picklist management file. Picklist Management file contains "Legacy Picklist" data only. The Odata API entity "Picklist" will only be filtered with the legacy picklist Ids which are present in the picklist management file, not with the MDF Picklist Ids.
An "MDF Picklist" is actually a "PicklistV2".
The correct query is:
/odata/v2/PicklistV2?$format=JSON&$filter=id eq 'PositionMatrixRelationshipType'
Picklist, PicklistV2, MDF Picklist, Legacy Picklist, ODATA , KBA , LOD-SF-INT-ODATA , OData API Framework , LOD-SF-INT , SF Integrations - EC Payroll, Boomi/ HCI, API , Problem