After analyzing dozens of faulty CX31993 implementations and reverse-engineering cheap dongles, three primary causes emerge:

The CX31993 integrates a 32-bit, 384kHz DAC with a charge pump. The block diagram in the datasheet shows a direct path from the DAC to the headphone amplifier (HP AMP) output. There is no dedicated external analog LPF. This means ultrasonic noise (from delta-sigma modulation) folds into the audible band as intermodulation distortion, manifesting as perceived brightness.