When you assign strongly typed values to an array, the type declaration will remain intact only as long as you do not add new array content:
$array = [Int[]](1,2,3,4,5)
$array.GetType().FullName
System.Int32[]
$array += 6
$array.GetType().FullName
System.Object[]
$array.GetType().FullName
System.Int32[]
$array += 6
$array.GetType().FullName
System.Object[]
Once you add new content using +=, the complete array will be copied into a new one, and type information is lost. You can work around this by simply strongly type the variable that stores the array instead:
[Int[]]$array = 1,2,3,4,5
$array.GetType().FullName
System.Int32[]
$array += 6
$array.GetType().FullName
System.Int32[]
$array.GetType().FullName
System.Int32[]
$array += 6
$array.GetType().FullName
System.Int32[]
As you can see, the type information is preserved this way.
