When does Puck follow and not follow his heart in Shakespeare's A Midsummer Night's Dream ?

We especially see Puck follow his heart when he reaches
out in sympathy to whom he thinks is Helena. Oberon has just witnessed Demetrius being
cruel to Helena in the woods and has commanded Puck to use the flower on Demetrius in
order to make him fall in love with her, thereby mending her broken heart.
Unfortunately, the only guidance Oberon gives Puck for finding Demetrius is that he
"shalt know the man / By the Athenian garments he hath on," not knowing that there are
actually two Athenian couples in the woods that night (II.i.269). When Puck finds whom
he think is the correct Athenian couple and sees the fair maiden sleeping so very far
away from the gentleman, he assumes that it is because the man is being cruel to her,
rather than that the woman is trying to preserve her maidenhood. Puck's heart genuinely
goes out to the woman in sympathy, and he chastises the man,
saying:

Churl,
upon thy eyes I throw
All the power this charm doth owe:
When thou
wakest let love forbid
Sleep his seat on thy eyelid.
(II.ii.78-81)

In calling whom
he thinks is Demetrius a "churl," Puck is calling him a low and insensitive person. His
chant is also promising that once the man awakes and lays eyes on whom Puck thinks is
Helena, love will never let the man sleep again. Hence, we see that Puck follows his
heart in amending what he thinks is a terrible situation and by doing what he thinks is
rescuing a maiden from cruelty.

In contrast, Puck follows his
prankster instincts rather than his heart when, after realizing he has enchanted
Lysander to fall in love with Helena rather than Demetrius, according to Oberon's
instructions, he brings Helena to Demetrius, but with Lysander in tow. After Oberon sees
how Puck has broken up a true-love relationship rather than mending a broken
relationship, he commands Puck to bring Helena to their part of the forest while Oberon
enchants Demetrius. Puck successfully does this, but Lysander follows after her. Puck
thinks it is great fun to now witness both men fighting over the same woman, as we see
in his lines, "Shall we their fond pageant see? / Lord, what fools these mortals be!"
(III.ii.115-116). Oberon accuses Puck of having brought Lysander with Helena on purpose
thereby inciting a ridiculous fight among all four Athenians, as we see in Oberon's
lines, "This is thy negligence. Still thou mistakest, / Or else committ'st thy knaveries
willfully" (III.ii.361-362). Since it can be said that Puck brought Lysander with Helena
so he could observe their fight, thinking it great "sport," we can say that in this
moment Puck was following his mischievous instincts, rather than his
heart.