Lysander was NO LONGER under a love potion at the end of Act III.
Lysander was erroneously given a love potion. The love potion works by making the person fall in love with the first person he sees upon opening his eyes. Lysander fell in love with Helena.
Upon realizing his mistake, Puck, once again applied the love potion to Lysander's eye lids and this time, it worked perfectly because Lysander fell in love with Hermia, the one he really loves.
