Perhaps you could try a different/longer compression plug, which might have less chance of slipping? Maybe this one:
https://www.chainreactioncycles.com/deda-elementi-expander-bung/rp-prod171842?utm_source=customer-emails&utm_medium=stock-email&utm_campaign=backinstock

Or have you tried using carbon past where the handle bars clamp to the steerer tube to give some extra grip?

Another thought: Have you tried using some sort of retaining compound on the headset bearings in case they are moving within the frame?

Might be worth trying something like in this video:
https://youtu.be/IQWYPxGicjU?t=268

Can you share some pics? What size did you get?

• Production & shipping was way faster than I expected, I think just over 2 weeks from payment including custom paint job. I could have built the bike much sooner had my separate BB not got lost in customs.
• The frame feels great, seems to ride like any premium carbon gravel frame, and the custom paint looks rad.
• The overall finishing seems good, with no paint in threads, bearing races pretty clean etc.
• I couldn’t see any obvious voids or terrible wrinkles in the carbon - but will report back if it snaps in half jumping off a curb.
• I’m quite impressed but he SRX groupset, shifting is crisp and levers feel decent.
• Elite wheels seem fantastic on the short rides I’ve done so far. Tubeless ready tyres popped right on with a track pump.
• The Juin Tech brakes, now set up and bedded in are solid. Not quite hydro feeling, but much much better than BB7s I've previously used. They could probably be made better if my cabling wasn't such a mess too, or just external routing.

• Threading compressionless housing through the bars+ stem + frame is ridiculously difficult. Compressionless housing is too stiff to get round the corners and it gets wedged in hidden corners of the bars. I’d estimate I’ve spent over 8 hours trying to get it set up right here, although that’s largely because I screwed up the first run. I’m also running 1x here, doing a mechanical 2x setup + compressionless housing would be extremely difficult.
• Seat clamp: the front bolt can’t be accessed with an Allen key unless you have a cut-out saddle, so fine-tuning the angle is a bit of a pain
• The bottom bracket pressed in worryingly easy. I could tighten the press-fit tool by hand, without a wrench. It’s not creaking yet, but I’m expecting that I might have to add Loctite retainer in the future. This is a Rotor BB not a VB one.
• Minor gripes: Wasn’t supplied enough bolts for all the bosses, the seat post doesn't slide that far down so I had to cut a lot off it to get it to the right height, where the cables exit the frame and attach to the brakes/derailleur causes quite sharp angles.

• I got the separate bar and stem as I wanted the option to swap out the bars to attach aerobars for really long distance rides. I also wasn’t 100% on the geometry, I just matched it to my road bike. In retrospect with the integrated cables swapping the bars out is something I’m going to try and avoid.
• I got the updated headset that Chris has mentioned in previous threads, this seems to resolve the issues others were having of not being able to preload the bearing. However, it definitely has not been perfect. I’ve battled it slipping and coming loose, not being able to preload sufficiently for several days now.
• Possibly the issue lies in the way is presses the C clip/dustcover into the bearing, it looks uneven and I’m assuming this then causes it to slip slightly under hard braking. If I’m careful with aligning the C pointed exactly straight ahead, arrange the cables to sit evenly spaced, and add a little carbon paste on the stem it seems to work.
• I’ve also got a 1-2mm gap like coffeebreak mentioned above. I think I’ll just cover it with silicon as removing the dust cover is an ordeal.
  I think long-term the solution is probably still to go with the FSA headset.