I've noticed that the preview is sometimes quite far from what it will really take. Here's an exemple taken in a room with some Lightning
The 24mp shot is actually quite disappointing. Try it with the default 6mp mode it will give better result.
If you want better results with 24mp camera use the gcam mods
24MP Selfie mode need a lot of light. Just like the 48MP mode with the primary camera. You're only supposed to use these mode in well lit environments
I don't understand all this criticism about the Rog 2 camera. Not only in this post but all over the forum. Here is the example of the selfie taken in a room with blinds completely closed. The same situation with 24 mpx and 6 mpx selfie cam. You will see the 24mpx image is less processed and has more detail. It's better in every way. The room in reality is much darker than it looks. Also the available light is coming from behind. Worst possible photography conditions. For this reason photos are highly processed by the camera app, look much brighter and lack in detail. Considering those conditions, physical limitations of the camera (small surface of the sensor and each pixel, diameter of the lens) I consider these pictures very good. The Sony sensor built in Rog 2 is really good. For this reason I prefer 24 selfie and 48 main camera above all other modes. For anything better I would not choose another phone but any Nikon DSLR with prime lens. Even the cheapest one makes much better photos than any phone.