If you find the solution, please let me know.
I’ve had the same thing from a brand new fully built 456 from On-One and I’ve checked pretty much everything:
Steerer tube not lifting out of crown assembly,
spacer under top cap and small gap between top of spacer and steerer,
stem bolts done up to required torque, then tighter, then tighter still,
load up top cap after tightening stem,
grease the steerer, de-grease the steerer,
The only thing left is replacing the headset, but it was brand new and did it FFS!
It’s almost a year old now so perhaps I should as it’s the only thing that lets the bike down.
edit ^^^ no one mentioned trying a different stem.