Designing for Accessibility

An object on the focus order for a screen reader is only active when the layer is shown. 

For your example when a user tabs the the button and presses enter/space then the layer will show and what is on that layer becomes active. However, if the object is nit directly after the button in the focus order  (i.e. it is before the button or a few items after the button) then that object will not be read immediately once the button is pressed.

There is a couple of times when this is not the case, if you have the layer properties set to prevent user clicking on the base layer, then the layer becomes modal and immediately takes focus, then it is dependent on how you have the focus organised for that layer. In a future update when Storyline add Dialogue layers these will behave the same way that they take immediate focus.

It is often difficult to retrofit solutions on to a build that had no thought for accessibility, for me I always work with the idea that the experience does not have to be identical it just has to be a good experience. You do not want the user to get lost and it may mean building in additional accessibility functions.