In my experience 25.4:1 denotes an open or possibly shorted antenna connection or a failed SWR measurement circuit. What happens if you bypass the ATU and transmit (or tune) into a known load? Do you get an expected SWR?
That SWR normally indicates an open or shorted antenna connection. It
is the maximum value the KX3 can display.
What happens if you tune into a dummy load with a known good coax jumper
connecting the KX3 to the dummy load?