SQL SERVER Convert BLOB data to STRING
The value truncates because you have not specified a length for the convert to varbinary. Without a specific length you get the default which is 30.

select convert(varchar(max), convert(varbinary, '1234567890123456789012345678901234567890'));
select convert(varchar(max), convert(varbinary(max), '1234567890123456789012345678901234567890'));


Result:

123456789012345678901234567890
1234567890123456789012345678901234567890