It seems like in the SQL issued by the system, the extension table is missing query hint such as WITH(READUNCOMMITTED). These query hints are vital for better performance, especially to avoid blocking and deadlocks. The captured SQL command is attached to this request.
Also while referring the extension table add 'AS' key word for consistence with the rest of the code.
The expected SQL command should be something like

SELECT TOP (@0)
List of fields from both base and extension tables ,.....
FROM "Cronusgb".dbo."CRONUS UK Ltd_$Sales Line$437dbf0e-84ff-417a-965d-ed2bb9650972" AS "Sales Line" WITH(READUNCOMMITTED)
JOIN "Cronusgb".dbo."CRONUS UK Ltd_$Sales Line$ef933c61-34d2-4d4e-ada4-6fdf714541ab" AS "Sales Line_e1" WITH(READUNCOMMITTED)
ON("Sales Line"."Document Type" = "Sales Line_e1"."Document Type") AND ("Sales Line"."Document No_" = "Sales Line_e1"."Document No_") AND ("Sales Line"."Line No_" = "Sales Line_e1"."Line No_")