Is it a bug that Enchant Blade is not cumulative? I don´t think so. But who knows? Most of buffs/debuffs seem to be cumulative. It would be much better if in case of non-cumulative buffs/debuffs only the last casted would appear. That way it would be quite obvious which one isn´t cumulative. Because there are buffs/debuffs we couldn´t really know (such as Deafening Crack).