Some bikes are suffering from a faulty Brake Light Switch sensor, usually the rear one. If the stop light is permanently turned on, this is likely the culprit. The fix is to replace the sensor under warranty, they are just defective from the factory. This requires removing the fuel tank.
The sensor seem to be self-calibrating, as such some stop and go action sometimes recalibrate the sensor and the problem fixes itself.
Push come to shove, replacing the switch is relatively easy (you just need to remove the fuel tank to get access). See this post on advrider by AdvRonski.
The switch itself is even available on Amazon (but will require a bit of soldering to adapt the connector, so prefer the KTM part: front p/n 64111051100, rear p/n 62111051000, the only difference between the two appear to be the connector). Only use the KTM parts: front p/n 64111051100, rear p/n 62111051000, the OEM ones you can source elsewhere are not using the same pressure to trigger.
A faulty Brake Light Switch sensor will cause the Cruise Control to disengage, not set, or not resume.