As others have said before, the question you should be asking is why your efficiency is not what you expected. It appears that perhaps there may be some lingering confusion on your part that is causing you to focus on the end result - the low ABV.
Here, you know your OG was 1.035. That should have immediately alerted you that your final beer was likely going to be a sub-3% ABV beer. The problem is not some mysterious infection or other type process. With an OG of 1.035 it would be impossible to get a 6 or 7% ABV beer. The alcohol comes from the conversion of sugars into alcohol by the yeast. Low sugar levels in the wort leads to low ABV levels, simple as that. That you did not immediately focus on the your OG (and thereby on your efficiency) indicates some confusion on the topic of gravity readings and their correlation with ABV. It also should not have been surprising that it bubbled just fine, as it clearly still went through a complete fermentation process--no issues there.
Looking at the numbers can help narrow down what the expectations could have been. Based on your NB kit, you could have expected an OG of around 1.044 (based probably on an average efficiency). Assuming that fermented down to 1.013 or so, you would have been looking at a 4% ABV beer. According to the best information I can dredge up online, it appears that your 10 gallons of maple water would have raised your OG by about 8 points. Again assuming average efficiency, you could have expected an OG of around 1.052, with potential ending ABV of around 5%.
Given that you ended up with an OG of 1.035 (and assuming your volumes were fine), and accounting for the fact that you probably got around 8 points from the maple water, then your sugar extraction from the grain was very poor. An OG of 1.027 from 8.5 lbs grain means you only got about 45% efficiency, which is not good. That is where you need to be focusing. Either something about the sap reduced your otherwise good efficiency, or your process is leading to poor efficiency.
Your description of the amounts of maple water you added leaves some of us scratching our heads a bit. I think it would be a good idea if you could give an overview of your mash and boil process so we can try to figure out what is going on. How much and at what temp was your maple water mash addition. How long did you mash and at what temp. How much maple water in sparge, what temp and for how long. What was your preboil volume. How long did you boil, what was your post-boil volume. How much top-off. Those sorts of details, along with any other measurements that you took (e.g., mash ph) would be helpful for us to help troubleshoot your poor efficiency issues.