He decapitated himself on his first evolution and he achieved the clarity during his second an hour or so later. during the clarity he learns his power runs along his spine but is concentrated at the top and tailbone hence he will regenerate from the spine or one of those points (which is why the tails act subconsciously and any electricity triggering his nervous system leads to rapid uncontrollable growths). considering he was on his first it would have been easier for his body to regenerate from the head instead of messing with the brain. also how the person thinks their powers work will affect their development (such as Ella saying jakes powers were more people focused and limited than they should be.) TLDR his body saw it easier to regenerate head down.