When you level up haste (and crit, mastery, hit) suffer from diminishing returns meaning 1 haste rating yields less and less haste percentage the more levels you take. Your DoTs gain extra ticks while keeping the same duration at certain haste percentage plateaus, but leveling up increases the amount of haste rating needed for each plateau even though the percentage stays the same.
Losing one tick on your DoT will mean fewer ticks, while more powerful, and overall most likely a decrease in overall damage from the DoT regardless of a few intellect upgrades.