I have exactly the same problem with my ATR fork. The bung is crappy.
I tightened the headset again this morning. I'll see how it goes.
This is with the longer, knurled bung I replaced the stock one with.
Which replacement are you using? I bought a Hope one and it's slightly too wide for the steerer.
@hippy started
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I have exactly the same problem with my ATR fork. The bung is crappy.