Hi @en_morty,
As I understand it that’s a notification more than a warning - ArduPilot’s EKF3 Lane Switching documentation says the EKF runs an instance per IMU, and if one set of sensors is misbehaving (or just noisier) it switches to another.
It may be worth doing a sensor calibration - if the notification persists there may be something wrong with one of the sensors on your autopilot board.
That should be possible using the Bridges page (requires Pirate Mode). We recently made some significant quality of life improvements to that page, so if you try out 1.1.0-beta.6 it should be extra pleasant to use 
Of course any feedback is welcome, and please let us know if something isn’t working as expected