Reproduction steps:
1. Open the attached project. Load scene "CameraTest" and enter a play mode;
2. The scene has two cameras, each one renders one cube. Child camera has depth 0, Main camera -1;
3. Press "C" to trigger the the script attached to "Main Camera". The script temporarily assigns a render texture (targetTexture), invokes the Render() method and sets back targetTexture to null;

Expected results: "Child Camera" has a higher depth order (depth=0) and does not clear any buffers. It is supposed to render the second cube based on the depth information provided by "Main Camera" (depth=-1).
Actual results: After activating the script, "Main Camera" is not drawing any longer. The buffers are not cleared and only "Child Camera" is rendering on top of the existing data.

Reproduced on: 5.6.0p3, 2017.1.0b2.
Not reproduced on: 5.5.3f1.

Reproducible both on Mac and Windows.