Jakub Voracek finished a 3-on-1 rush to make it 3-0 at 14:01.
The Islanders started their comeback when Dal Colle made it 3-1 at 7:53 of the third with a shot from the left circle for his first goal in 32 games, dating to Feb. 6, 2020.
Sebastian Aho scored his first goal since Jan. 7, 2018 with a wrist shot from the right circle to make it 3-2 at 13:53.
Oliver Wahlstrom tied it 3-3 with his third goal in four games at 14:27 off a seam pass from Leddy.
"I think after the second we just talked about let's just move forward and get to our game and give ourselves a chance here," Islanders forward Josh Bailey said. "Even if tonight doesn't go the way we wanted, let's at least finish strong and find something to build off of.
"We played a good, hard third. One play turns into the winning goal for them. It's unfortunate, but that's the way the game goes sometimes."
NOTES: Flyers defenseman
Nate Prosser
had an assist on Giroux's goal, his first since Feb. 27, 2018. ... Leddy has scored 233 points in 493 games with the Islanders, passing
Kenny Jonsson
(232) for fourth all-time among their defensemen.
Denis Potvin
is first with 1,052.