The Ruling on Using a Camera App as a Mirror
If it is said:
We did not take a picture, as it is not fixed and disappears when the app is closed.
One could say to that:
The imitation is still fulfilled. What you see on the screen is not a reflection, but automatic image-making.
The image being permanent is not necessary for imitation to be fulfilled; creation itself is the condition for that.
Based on this, using a phone camera as a mirror is not allowed, and it is a major sin.
Important Note
If it seems strange to you that the app creates pictures without pressing the capture button, this indicates a lack of understanding of the nature of photography and how these devices work, and does not mean it is not considered photography.
The advice is: Do not speak about what Allaah ruled, without knowledge. This matter concerns a major sin.
Therefore, please consult experts before issuing a ruling on a purely scientific matter related to these modern devices.