I figured out what the problem is

Hello,GriV as I thought - the problem is in the output filters of the AY itself and throughout its further wiring (up to the amplifier itself). AY tries to generate an ideal signal (1-step step) but in the end due to filters and these fluctuations occur. Typical filter frequency - 22 kHz (44 kHz for the signal quantization frequency) - apparently at this frequency the signal is cut.