Select Into removes IDENTITY property from target table
The behaviors you're curious about are explained in the documentation:


  When an existing identity column is selected into a new table, the new column inherits the IDENTITY property, unless one of the following conditions is true:
  ...- The identity column is part of an expression.
  ...If any one of these conditions is true, the column is created NOT NULL instead of inheriting the IDENTITY property.


And in this thread from UC:


  Metadata is determined based on the source column and expressions used
  in the SELECT list. Below are the rules:
  
  Any expression that uses a built-in function like SUBSTRING, LEFT, RIGHT etc (except ISNULL) for example is considered as NULLable by the
  engine. So if you use CAST(somecol as char(8)) then the expression is NULLable.
  ...
  So to make an expression or column in the SELECT list not null then use ISNULL around the column or expression.