To reproduce:

1. Download and open attached "bug.zip" project.
2. Open "MSAA_perf_warning" scene.
3. Select "Back Cam" GameObject.
4. Change Its Viewport Rect X or Y values to be anything but not 0. For example set Y to be 0.75.
5. Notice that "Tiled GPU perf. warning" warning appears in the console.

Expected result: No warning appear in the console.
Actual result: "Tiled GPU perf. warning: RenderTexture color surface (1080x1920) was not cleared/discarded. See TiledGPUPerformanceWarning.ColorSurface label in Profiler for info" warning appears in the console.

Workaround: Have only one camera in the scene with "Allow MSAA" enabled.

Reproduced with: 2018.2.0a1, 2018.1.0b6, 2017.3.1f1, 2017.2.1p3, 2017.1.3p1, 5.6.5p1. 5.6.0a1.

Notes: In 5.5 where allow MSAA option was not yet introduced everything works as expected. Warning seems to appear only when Platform is set to Android. Issue does not reproduce if platform is set to WebGL or Standalone. In some cases, when ViewPort Rect values are set to be specific numbers, for example set Y to be 1, then warning does not appear, but when scene view is focused error appears "Screen position out of view frustum".

By design: warning to warn about potential performance cost on mobile tiled GPUs, in Editor warning can be suppressed by disabling Graphics Emulation.
more info: https://docs.unity3d.com/ScriptReference/RenderTexture.MarkRestoreExpected.html