I had something similar happen on a bike. After a lot of fuss, it turns out that the expander plug wouldn't sit tight in the steer tube, so when I tightened it down it would pull out a bit. So I never got proper preload. It caused the bike to shudder like mad under hard braking, genuinely terrifying.
I replaced the cheapo expander plug with one of those long Specialized plugs. That thing isn't going anywhere. Solved the issue for me.
Not sure if this is what you're experiencing but it sounded similar.
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