9 times out of 10 there is more to the story. Your problem is very typical of what happens AFTER grips have been changed or the brake lever removed for some reason and the little plastic button on the front brake light switch gets broken off. So, have you opened up the switch housing, changed grips or fiddled with the brake lever? Check to see if a very small movement of the front brake lever lights the brake light. If it does, the cruise is behaving as designed, either brake light switch makes it disengage. The next question is why is the brake light switch "out of rig"? The most frequent reason is the little plastic button missing, but there is also a little V shaped spring inside the switch housing and if it is gone, the switch gets pushed back inside the switch housing and again activates the brake light with little or no lever movement.
For those who lose their cruise on bumps, the OWNERS MANUAL explains that among other reasons, an abrupt increase in engine rpm will also disengage the cruise. Having the rear wheel lose ground contact, especially when the engine is under load (climbing a hill or pushing against a strong headwind) would allow rpm to increase rapidly, even if only for a moment causing the cruise to disengage.