Diagnosis
The cliff sensors are interpreting the dark rug as a drop-off. This is a known limitation of infrared sensor technology with low-reflectance surfaces.
How to Fix It
There is no setting to disable cliff sensors on the W90. Your only options are physical. Place a light-colored sheet or a thin, solid-color mat under the rug in the robot's path. Alternatively, use the included magnetic boundary strips to block off the area entirely.
If That Didn't Work
Clean the cliff sensor windows on the underside. Dust can cause erratic readings. Use a dry cloth. Ensure the rug is not frayed or has long fibers that physically trigger the front bumper, which would cause a different 'collision' behavior.
Heads Up
Covering the sensors with tape is not recommended. The robot will lack drop protection on actual stairs, creating a safety hazard.