In release R2015a, the ViewCanvas constructor uses the wxGLCanvas constructor without explicitly asking for any depth buffer precision. This can result in a GL context created with no depth buffer. In this case, then, the OrbitView rendering is incorrect because there is no depth sorting or testing.
Attached are screenshots demonstrating the issue and a simple patch to fix it. The patch changes the GmatGLCanvasAttribs array, adding in a pair of values specifying a WX_GL_DEPTH_SIZE of 16 to request a depth buffer explicitly. This results in the correct behavior, at least on the GL stack I am using.
GMAT R2015a; CentOS 7.2.1511; Gallium LLVMpipe renderer; see attached files for detail.