depth test auto-disabling for larger windows

I've been modeling 3D fractals using Marching Cubes and came across a weird issue. When rendering in a small window, everything works as expected, but at larger resolutions my model would suddenly glitch out with weird stripes and wrong lighting. After a bit of tinkering I found the cutoff point to be a horizontal size of 640, vertical size doesn't seem to matter. When opening or resizing a window bigger than this, everything is fine for about 10 seconds, then the depth test suddenly disables and the artifacts appear, every time. I fixed it in my app by moving ofEnableDepthTest from setup to draw, but definitely seems like a bug somewhere. I'm using 0.9.0 on a Radeon 7670M with latest drivers.

Reproduce the issue with Marching Cubes in a window wider than 640, then compare the depth-test state after setup and during draw, where ofEnableDepthTest is currently used. Confirm the cause of the delayed disable and verify that depth testing remains enabled without requiring an application workaround.