Correctly debugging switching (DC-DC) boost converter with Oscilloscope
From the problem description, two possible hypotheses come to mind:


There is a ground level mismatch: The DC-DC boost output has a ground that is different from your oscilloscope's ground
The probing was done with no LED load, so the boost circuit is attempting to generate the highest voltage it can. How high this can be depends on the circuit design, of course.


With some additional detail on the boost conversion design, additional hypotheses, or a definitive answer, might be possible.