Several things could be at play here.
First and foremost, did you test your thermometer before using it to boil the syrup? Thermometers vary wildly, especially depending upon barometric pressure, and testing is more important the higher you cook. To test yours, boil some water in a saucepan and place your thermometer in the boiling water and take a reading. Does it read 212 degrees (F)? Most don't. If it reads 215 (for example) you know that the hard-crack of 300 degrees will actually be 303 degrees for your thermometer at that time on that day. If it reads 200, then you would subtract 12 degrees from your final temperature target.
My second concern is your altitude. Temperatures for cooking candy are based upon readings taken at sea level. Higher altitudes make progressively profound impacts upon thermometers! Here is an online calculator that will help you establish precisely what temperature water should boil at in your area, depending on the barometric pressure and altitude. Once you know that, then you can adjust your thermometer appropriately.
http://www.csgnetwork.com/h2oboilcalc.html
Sugar-based solutions will definately burn at higher heats, but it has not been my experience more than a couple of times over the years. I cook mine over medium to medium-high heat typically, perhaps a little slower if the humidity is a little high (over 65%). But I'm really thinking here this has more to do with where you are, more than what you are trying to do.
What are you making here?
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