When the belt skips (what happens when the Z-Axis hits the board) then that Z axis value will no longer be accurate. It’s open-loop control, the controller only knows that it told the stepper to move the axis, so it now believes the axis has moved. It does not know that it did not move.
It does sound like that is what is happening in you first video; could upload the gcode used? It could be that one hole is set to cut deeper and the bit hits the metal strap.