Sorry you're having this issue! We hope to have it fixed in a future update.
If you're playing on a Google Pixel or OnePlus device, the auto-brightness and smoothing features could be the cause of this.
Try turning these features off and refrain from changing brightness manually during the song.
If this did not solve the issue or if you're playing on a device that's not a Google Pixel or OnePlus, please contact us at firstname.lastname@example.org so we can investigate it with your help.