Has the index() function changed recently?

I have used the index function reliably over the years to pull multiple items from an array (in particular using index(wherecontains())  however I'm now seeing a very odd behavior and I wanted to know if something has changed.  In the epxression below I would expect the index function to return a series of nulls followed by the number 3.  Instead it reutrns the following:  {PK: 1}, {PK: 2}, {PK: 3}, {PK: 4}, {PK: 3}

Using the index statement with a scalar value for the index, ie: just the number 9 instead of the array, causes the expected behavior to occur. 

a!localVariables(
  local!list: {
    {PK: 1},
    {PK: 2},
    {PK: 3},
    {PK: 4},
    {PK: 5},
    {PK: 6},
  },
  {
      fn!index(local!list, {7, 7, 7, 7, 3 }, {})
  }
)

Does anyone have any insight about what's happening here?

Note: For more fun you can replace the index statement with:

fn!index(local!list, {-1, 4, -1, 4, -1, -2, -1, -1 }, {})
 ==> 
[PK:1]; [PK:4]; [PK:2]; [PK:4]; [PK:3]; [PK:4]; [PK:5]; [PK:6]

fn!index(local!list, {7, 8, 10, 203 }, {})
 ==> 
[PK:1]; [PK:2]; [PK:3]; [PK:4]


fn!index(local!list, enumerate(10), {})
 ==>
[PK:1]; [PK:2]; [PK:2]; [PK:3]; [PK:4]; [PK:5]; [PK:6]; [PK:3]; [PK:4]; [PK:5]

}

)