Cidery flavors are acetaldehyde. Also described as green apple flavors. Sometimes that's due to taking it off the yeast cake too quickly, or underpitching yeast. It can be due to too much simple sugar, as well, and that's an interesting cause, too. When yeast ferments too much simple sugars, it actually loses the ability to break down and ferment maltose. The yeast naturally secrete an enzyme which breaks down maltose (malt sugar) into monosaccharides, which are easily fermented. When you use too much simple sugar, the yeast hit that stuff first, and after awhile, it just stops secreting the enzyme and essentially "forgets" how to ferment maltose. So, while some styles REQUIRE simple sugars (like some Belgians, Double IPA's, etc) in order to ferment down dry enough to be drinkable, too much simple sugar can do exactly what you describe. The other problem is that since maple syrup (like honey) is so fermentable, the yeast really consume the heck out of it. When the yeast is done with it, most of it (including the flavor) is gone.
I just read through the syrups section in Randy Mosher's book, Radical Brewing, which I just got for Christmas. (HIGHLY recommend this book if you're looking to experiment. I've only read through parts of it, and it's already helped me redesign one of my beers to help with some efficiency issues I was having.) The subject of maple reminded me of it, and I'm glad I did. He recommends adding the maple syrup to the secondary fermenter. (In the secondary, most of the alcohol has already been created, so there's not much for any bacteria to get a hold on.) The yeast is also mostly spent, so most of the maple flavors will remain in your beer. However, it seems to me you'd be running the risk of adding bacteria along with the syrup. I agree with him that this would be the best time to get the most flavor, and you won't have to worry about acetaldehyde, because your yeast won't be doing a TON of fermenting of the syrup. Finally, he recommends that you use the B grade maple syrup. It's not as refined as the A grade "Fancy" stuff, and will leave a lot more flavor in your beer.
He also suggested that if you want to enhance and extend maple flavor in your beer, the spice fenugreek has such a "maply" flavor that it is often used as a substitute for actual maple in the cheaper fake maple syrups. He says you can add this instead of or in conjunction with your maple syrup to the secondary.
Good stuff!