When the device is modeled as GnSNMPDev serial number is populated but the Manufacturer field is empty.
When the device is modeled as Rtr_Cisco the serial number is empty.
Cisco ASR devices missing Serial Number in Spectrum
All supported DX NetOps Spectrum releases
The device doesn't provide the MIB OIDs and values that provide the serial number value.
The Attribute deviceSerialAttr (attribute ID 0x3d0063) relies on Attribute chassisid (attribute ID 0x210b5d; OID 1.3.6.1.4.1.9.3.6.3.0) for the serial number value from Cisco devices.
Non-Cisco devices using GnSNMPDev Spectrum reads the value of the entPhysicalClass OID (1.3.6.1.2.1.47.1.1.1.1.5) value from the Entity MIB on the device. If it finds a value of chassis(3) Spectrum reads the corresponding instance value of the entPhysicalSerialNum OID (1.3.6.1.2.1.47.1.1.1.1.11) value. If populated it populates the Serial_Number attribute (attribute id 0x10030) with the value.
Not sure what values are seen for a devices MIB OIDs?