A proper charge from any cav will nearly wreck spears. What you are seeing is a differential in the post-charge fighting. Heavy cav has armor, spears lacking armor penetrating weapons. Hence while they have an advantage against light/medium cav and can eventually win out, they have too hard a time hurting heavies.

The shield bug has NOT been fixed yet. CA didn't even know about it til long after 1.1 was released.