Disabling "Enable MSAA" on cameras has no effect (5.6.3p4)

I'm trying to disable MSAA on one of my cameras, keeping it at 4x MSAA for another camera rendering above for performance reasons.

However, unchecking "Allow MSAA" appears to have no effect - if I've set 4x MSAA in the Quality Settings, that is what all my cameras default to, and cannot seem to opt out of. Is there any way around this?

N.B. I know I could use another post-processing anti aliasing effect, but MSAA is the only AA technique that works for our project's purposes on certain small-scale details.

 

What is your target platform(s)?

 

This is on a PC targeting desktop as well. I haven't tried exporting for other platforms, but it is the case both in-editor and in-build.

I've figured out the issue is always with multiple cameras. "Allow MSAA" works as expected with a single camera, but there appears to be no way to do this with multiple cameras, as if one of them has MSAA enabled it will override all cameras to have MSAA applied.

I've attached a small Unity project demonstrating the issue (5.6.3p4). 2 cubes are rendered, one by a camera with MSAA enabled and one without, yet both result in anti-aliased results unless both have MSAA disabled.

I've tried following solutions that involve rendertextures with differing anti-aliasing levels blitted in sequence, but it gets out of hand quickly with multiple cameras / clear flags and transparency.