One of the keys in determining the optimum printed circuit board layout is to understand how and where the signal return currents actually flow,  Most designers only think about where the signal current flows  and ignore the path taken by the return current.

we must understand how the high frequency current flows in a conductor.

First, the lowest impedance return path is in the plane just below the signal trace because it gives the lowest inductance path. It also produces the smallest current loop area possible.

Second, because the high frequency currents can not penetrate a conductor, and therefore, the high frequency currents on the surface of all the current conductor.

In affect on all frequencies above 30 MHz to 1 oz.

Therefore, a plane on a PCB really two conductors is not a conductor. There will be current at the surface of the plane, and there are different currents or none at all currents below the surface plane.
These discontinuities lead to the return flow of the larger loops, which increases the radiation from the board, and increasing waves of distortion.
The best solution to this problem is to avoid crossing a plane split with the critical signal and the signal should be passed on to the bottom layer because the signal is close to the ground.