If I'm understanding right, you've got an upper headset bearing compression ring that you don't trust is dimensionally stable, and the headset keeps coming loose.
It is true that janky plastic compression rings exist and can cause such problems. In most cases it should settle down and hold adjustment better after a few repeats of the problem. Give it a touch more preload than you otherwise might.
I've met a few Canyons where the factory assembeld steerer compression plug isn't adequately tight in the steerer. The compression plug metal-on-metal wedge surfaces weren't greased, with the effect that its holding power was inadequate even though it seemed like it was torqued properly. (Many carbon steerer plugs can do this). It wouldn't surprise me if you're having some amount of that problem, and it's contributing to the headset loosening because when you go to adjust the headset, you're not preloading it as much as you think you are because the plug is slipping up. So one thing to do is before you go any further, pull the stem off and check whether a gap has opened up between the top of the steerer and the plug. (Note that once the stem is secure, this is not the problem, but it could contribute to the problem by making adequate preload impossible to achieve before the stem is secure.)
