This seems to be a bug in the slicer. I’m seeing the same issue in OrcaSlicer if the skirt height is set to more than one layer (even if the skirt is disabled). This makes sense if a skirt is used (can’t print a continuous spiral if you have to switch between printing the object and the skirt), but not if it’s disabled.
What slicer are you using? If it’s a PrusaSlicer fork, this is likely the same bug.
Sure, the skull is biodegradable. But didn’t you know that large-scale human farming is responsible for the vast majority of CO2 emissions?!