How to adjust this headset?

Dear All,
I inherited this small Bombtrack gravel bike and I am trying to set it
up for my kid. I have trouble adjusting the headset. As you can see in
the pictures it does not have the standard star nut but what seems like
an expander that protrudes from the top of the steerer. Nevertheless I
tried to adjust it by adding spacers and tightening the cup bolt. The
problem is that at around 0.8 Nm the bolt skips and does not get tight
enough (the spacers rotate by hand). I tried to use a longer bolt or
removing a spacer and in both cases the bolt gets tight and bottoms out
without preloading the headset.
Am I doing something wrong or is there any issue with the fork? Any
ideas will be appreciated.
Many thanks in advance
Dimitris