KiCAD: Cannot see traces on a PCB side which has been filled with copper

Try pressing B. That will re-draw all fill zones on the board.

Pressing b will Fill or refill the zones. Usually, I copper fill the board after the routing is done. Or you can turn off filled zone by clicking the Do not show filled area in zones button on the verticle toolbar on left.

You can also have the same effect when you run the DRC check, refilling one of the steps in the check.