It shouldn't slip if you hold it , it should be firm enough to resist slipping.Only if I hold, I can't work out what would hold it while riding. You mentioned earlier putting some insulating tape around the axle might help. I'll give that ago.
If I can get a new one that'll be the better thing to do but in the meantime some tape and a clean will be tried,It shouldn't slip if you hold it , it should be firm enough to resist slipping.
The plastic spline can and do lose grip , you have reglued your one so it may be lose because of that . Tape can be used to increase the axle diameter thus giving the spline something to bite into for grip which is more pliable .
One will need to degrease /clean the axle before applying tape .
If you look at the photo I posted I can't see that being possible.When fitting a PAS I never fit the disc to the axle , I prefer to bolt the disc to the inside of the smallest chain ring . There is no way then that it can ever move bar by crank rotation.
One then has to reverse the sensor head bracket so it is cranked out more to the chain ring , unscrew the sensor and screw it on the opposite face .
One would have to drill a new hole for the LED to poke thru. If the sensor gap is more then 1mm , I will if need be use a thin piece of 1mm /2mm ali shim behind the sensor head to move it nearer.
I was able to look at the disc whilst cycling and it moved as it should. I'm going to remove the brake sensors and try the bike without them. As most of the time the motor cuts out when the brakes have been used, It can't do any harm. Maybe the speed sensor is faulty.You could mark the disc and the axle with a paint pen/tippex/marker pen to see whether there is any movement. Then you could confirm if there is a problem or not, before spending any money and more importantly introducing a potential new variable into the equation.