Puzzle says forced mate is incorrect

en.lichess.org/training/1955

In this puzzle, I played Nf5+, and after Kg8, I played Qh6 with an unstoppable checkmate. I see there are more forcing/quicker ways to solve the puzzle, but to completely fail me for finding a forced checkmate seems harsh.

That's the way tactics work - you don't have to find very, very good moves. You have to find the *best* move. If there is a quicker forced mate you obviously played a very good move, but simply not the best.

I know this is sometimes a bit disappointing, but it trains the chess-eye for sure! ;-)

Most puzzles will say "Good Move, but try to find a better one." if you find a slightly worse solution(in this case, the solutions were actually equal in result)

Well after Ndxe7 mate follows the next move. After Qh6 black can give three checks. So the mate in three turns into a mate in six. So imo it is perfectly understandable that this variation is not accepted as a solution.