Most of the Islanders had gone to their locker room before a lengthy video review was initiated and ultimately determined that Hoffman was onside prior to his goal.
"It was extremely deflating twice, I guess," Islanders coach Lane Lambert said.
Sam Montembeault made 27 saves for Montreal (21-27-4), which tied the game twice in the third period and ended a four-game losing streak (0-3-1).
The Canadiens were playing their first game since Jan. 31.
"I was actually really happy with the effort today," Montembeault said. "That was their fourth game after the break and that was our first, but that didn't show. I think we played with intensity and with pace for 60 minutes. I gave up a little bit of a bad goal at the end of the second, but we didn't get down. We came back in the third, I mean, we came back twice."
Brock Nelson had a goal and an assist to extend his point streak to 10 games, and Semyon Varlamov made 27 saves for the Islanders (27-23-6), who have lost two straight after winning their previous four.
"I think we've got to do a better job, especially playing with the lead," New York defenseman Noah Dobson said. "We've got to be smarter the way we manage pucks, closing quicker on guys. We gave them a little too much time and space to get some movement, and obviously that wasn't good enough."