Ultrasound Vegetable Glycerin homogenization
Depending on intensity, ultrasound can produce intense heat at some locations. In fact, cavitation can induce temperatures far exceeding 5,000 K. Even a small ultrasonic room humidifier can burn through a plastic comb held in the upwelling stream. The sound intensity is decomposing the oil and/or propylene glycol.

Try lower frequencies and lower sound intensity. You might experiment with different shape containers, as well, to avoid focusing the sound into a high-intensity spot.