It is still good. Issues that could have caused it not to seal, something on the rubber part of the lid, temperature of the syrup not hot enough or something on the top surface of the jar. There could be other causes too. Just be glad you found it early. Put it in the fridge and use it next.
As long as the syrup was at proper density, maple syrup does not "go bad". It may get some mold on top, but if some ever does, heat it back up to 180-190F, skim the mold off and the rest is good.