It looks like there is an INSTEAD OF UPDATE trigger on your table. To find out the triggers associated with the table are executed as shown below.
In the object explorer, navigate to your table name and click node triggers.
or use the code below
SELECT sysobjects.name AS trigger_name ,USER_NAME(sysobjects.uid) AS trigger_owner ,s.name AS table_schema ,OBJECT_NAME(parent_obj) AS table_name ,OBJECTPROPERTY( id, 'ExecIsUpdateTrigger') AS isupdate ,OBJECTPROPERTY( id, 'ExecIsDeleteTrigger') AS isdelete ,OBJECTPROPERTY( id, 'ExecIsInsertTrigger') AS isinsert ,OBJECTPROPERTY( id, 'ExecIsAfterTrigger') AS isafter ,OBJECTPROPERTY( id, 'ExecIsInsteadOfTrigger') AS isinsteadof ,OBJECTPROPERTY(id, 'ExecIsTriggerDisabled') AS [disabled] FROM sysobjects INNER JOIN sysusers ON sysobjects.uid = sysusers.uid INNER JOIN sys.tables t ON sysobjects.parent_obj = t.object_id INNER JOIN sys.schemas s ON t.schema_id = s.schema_id WHERE sysobjects.type = 'TR' and s.name = 'your table name'
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