OK, since you pulled it apart for service that opens up other options.
The most likely is you put the bearing retainer in upside down.
Next is the possibility that you forgot or mis-installed the centering cone, or there's a spacer (not the stacking spacers) misplaced or missing.
If you didn't have the problem before taking it apart, you can probably rule out a bellmouthed headtube. Besides these usually cause very subtle movement, not what you're describing.
Take it apart sown to removing the fork, and start fresh checking every step along the way, then recheck by pushing the centering cone down manually with a spacer before installing the stem.
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