If you show a screen, it shows on the screens layer, which by default is above things like character sprites on the master layer. This is true of all screens, but you might not have run into problems before if you weren't using full background images in the screen. It can also show on top of the dialogue depending on the zorder (higher zorder goes on top of screens with lower zorder).
Ah I see, thank you! You're right, there is probably some kind of layering or zorder issue somewhere. I also have a hunch now that one of my screens being modal might have also had something to do with it.
I ended up using an alternative method to acheive the effect, but thank you for your help! I might still come back to this in future for other projects.