This issue was identified as a random system taking the GUID of the 'x64 Unknown Computer (x64 Unknown Computer)' record. As a result it was now a known GUID; as we were only deploying Task Sequences to the Unknown collection none were made available.
'x64 Unknown Computer (x64 Unknown Computer)' record
'x86 Unknown Computer (x86 Unknown Computer)' record
To get the GUID of your unknown systems open SQL management studio and run the following command:
--Sql Command to list the name and GUID for UnknownSystems record data
select ItemKey, Name0,SMS_Unique_Identifier0 from UnknownSystem_DISC
Using the returned GUID (SMS_Unique_Identifier0) we can find the hostname that has been assigned the 'x64 Unknown Computer (x64 Unknown Computer)' GUID by running the query below.
--x64 Unknown Computers
select Name0,SMS_Unique_Identifier0,Decommissioned0 from System_DISC where System_DISC.SMS_Unique_Identifier0 = '##Enter-GUID-Here##'
The query returned the hostname GUID and whether it is present within the SCCM database. A '1' implies the record is deleted and will be purged from the database. We saw a '0' imply the opposite.
Deleting the 'x64 Unknown Computer (x64 Unknown Computer)' record and recreating the record through the steps below had limited success. The issue was resolved until the new GUID created for 'x64 Unknown Computer (x64 Unknown Computer)' was used again.
delete from UnknownSystem_DISC where ItemKey in (##ItemKey##)
CreatedUnknownDDR Change the entry to 0
Restart SMS Executive this then recreates the Default x86 and x64 unknown collections
Microsoft has since release a Update Rollup KB4019926 which other sources were hailing as the fix.
After applying the Rollup we were still receiving reports of the Unknown GUID being assigned to systems. It was identified that when a build used the "Previous" button within WinPe (after a dependancy failure or simply to refresh task sequence policy) the task sequence would take the Unknown GUID still.
What was not highlighted in the documentation for the Rollup was the requirement to either recreate or update distribution points with the current Boot Image and then if you use USB boot media recreate this.