I had exactly the same issue! But with a M8 Spektrum pro. But it was the receiver here. Took me two races to change everything and as a last thing I changed the receiver. I know it is super hard to check it because you cant look at the LEDs when you drive it
You write that the car carried on at full speed. This is very unusual because the fail safe the spektrums have should bring it to neutral. So this is at least strange.
You have a KO and the Ko module is quite big and it is very easy to put a frsky in it. Take some time and the frsky module and you can put it in with servo connector plugs (without the plastic around) so if you dont like it it is very easy to remove. But this should be the easiest, cheapest and best solution if it has something to do with the module or receivers.