all the code inserting the data has to use the same datatype : NVARCHAR or NCHAR.
code as simple looking as
try this code example out and see what happens if you mix varchar and nvarchar implicit conversion into the mix:
declare @var varchar(10),
SELECT @var = '??/??' ,@nvar = '??/??'
SELECT @var = N'??/??' ,@nvar = N'??/??'
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