Protect first LED (W2812b) in line from voltage spikes?
I would blame the interaction between the fast PWM current drawn by the LEDs and the wire inductance to the adapter, or the adapter itself misbehaving on transients.

You can try putting an electrolytic capacitor at the power terminals on the LED strip. Something like 470µF, or whatever you  have lying around, preferably with ESR in the "not too high but not too low" zone, like 0.1-1 ohm.

If that fixes the issue, then my diagnosis was correct... If not, check the voltage with a scope to see if your supply does indeed misbehave.