I have been making this recipe for the past 10 years for Christmas. This year every batch has turned out bad. By bad I mean it doesn't have that brittle snap when I break it. The brittle is soft and dull looking not shiny. I have always cooked the sugar, butter, maple syrup and water to 290 F for a softer crack and then stirred in the walnuts. I did test my candy thermometer and it was off 15 F, I got a new thermometer and tried again cooking it to 290 F but got the same results. I make sure to only stir the mixture in the beginning until the sugar is dissolved and then leave until I mix in the walnuts. After I mix in the walnuts I pour it onto a silpad and let is sit on the counter.
You are absolutely right to use a candy thermometer; when used properly, it is the most reliable way to get good candy.
What happened here though is that your recipe is not optimal. The brittle is soft, because you took it to a stage called "soft crack". If you want it to be hard, you have to take it to "hard crack", which charts place at 300 to 310 F.
The reason it worked the past 10 years was probably either measurement error from the old thermometer, or some consistent handling error, such as not cooling the mixture quickly enough after the thermometer reached the 290 F, so it had a chance to heat up more even after being removed from the stove.
@rumtscho has probably got it, but since you say it's not shiny, I do wonder if your brittle might have crystallized and become a very hard, crumbly praline instead. Since maple syrup is primarily sucrose, the same as table sugar, it does nothing to reduce the risk of crystallization. If your walnuts this year had a bit more powder than usual (for example), that could have set off a chain reaction.
So you might consider adding a bit of corn syrup (which is primarily glucose) as an added bit of insurance.