Date & Time: Any time
Frequency: Randomly every few seconds
Area and coordinates: Anywhere, but my specific instance is in New Gridania
Steps:
1. Find someone performing with a guitar mode other than Overdrive.
2. Change window focus (ie, tab out) to anything other than the game
3. Random notes will play as if in Overdrive, including swapping the animation if the player is using Muted, Power Chords or Special.

It's noticable on Windows when the client is both Windowed or Full Screen.
I have 16 GB of DDR3 RAM, and a Radeon RX 570 (1GB) graphics card.
A friend with the same issue also has 16 GB of DDR4 RAM, and a NVidia Geforce GTX (3GB) graphics card.
Another friend who DOESN'T have the same issue has 8 GB of DDR4 Ram, and an NVidia Geforce GTX 1650 (4GB) graphics card.