Gradient, colouring and effect-error
The operations weren't particularly complex, a shape won't change fill between gradient, colour or even empty. The appearance panel is empty, and still there's no possible to change or remove the fill. I tried adding a gaussian blur, after which the entire figure disappeared. I thought it might be a preview issue so I exported it, but it was gone there as well. I attach a screenshot. Right before it happened I tried adding a complex clipping mask (but undid it).