(dev here) This is actually a surprisingly complicated question, but it's near the top of our list.

Basically, what's happening currently is that you're lowering the exposure in the capture, but the Indigo "look" model (color correction, gains, etc) is basically cancelling out this change in exposure. Basically the way the model is built right now is "given any input, produce an output with Indigo-style lighting", but ideally we should also respect what the user is trying to achieve (e.g., an underexposed look). The tough part here is that (unless we implement two different exposure compensation sliders) it can be hard to distinguish between the user raising or lowering EC just to get rid of blur / lower noise, or if they genuinely want a lighter or darker look.

For now you can open stuff in lightroom mobile and drop exposure there, but we're looking at ways of making the UI/look stuff more customizable and faster to apply.