Board shipped with wrong component values for R2 and C4. These two components form the snubber circuit from the output of the amplifier chip to ground and are needed to suppress high-frequency oscillations. The board comes with 10 pF for C4 and 220 ohms for R2. A quick look at the LM386 datasheet says C4 should be 50nF and R2 should be 10 ohms. After replacing these two parts with the desired values, the fluctuations disappeared and the board works normally.
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