Symptom. A predicate on an indexed column scans instead of seeking, with nothing in the query text to explain why.
Cause. AccountCode is VARCHAR. The literal is N'...', which is
NVARCHAR. NVARCHAR has the higher datatype precedence, so SQL Server
converts the column rather than the literal. A converted column is not
SARGable, so the index cannot be seeked.
Fix. Match the literal’s datatype to the column’s.
Spotting it in the wild. CONVERT_IMPLICIT appears in the plan’s seek or
scan predicate, and the plan carries a PlanAffectingConvert warning. This is
the most common cause of a mysteriously slow parameterised query arriving from
an ORM that sends Unicode string parameters by default, which most of them do.