How is it possible that a fragment is generated, passes the depth test but isn't written to the current render target?

This is the pixel history I see if I capture a frame in RenderDoc:

The fragment should pass the depth test (same depth of the corresponding pixel in the depthbuffer and depthfunc = Less equal), blending is not enabled, stencil test is not enabled, but the new color is not written.

What am I missing? Is there some other state I'm not taking into account?