TraversableSerializer does not perform a deep copy of the elements it is traversing

I had an issue where the state in my rolling window was incorrectly being maintained from window to window.

The initial state of my window looked like this:

Map[Key, MutableValue] = {("A", Value(0)}, ("B", Value(0)}

Then in Window 0 I update the state so it looks like this at the close of the window:

Map[Key, MutableValue] = {("A", Value(1)}, ("B", Value(3)}

Then at the start of Window 1 the state looks like it did at the end of Window 0:

Map[Key, MutableValue] = {("A", Value(1)}, ("B", Value(3)}

when I expected it to look like the initial state:

Map[Key, MutableValue] = {("A", Value(0)}, ("B", Value(0)}

It looks like TraversableSerializer is doing a shallow copy of the elements in the traversable instead of a deep copy