Thankyou, I found out that this was recently implemented by Google on Chrome 73. Chrome controls the multimedia keys on connected devices, and this also affects the touch bar.
If anyone else experience this same problem on Chrome or other chromium browser, try disabling it.
- Load chrome://flags/#hardware-media-key-handling in the Chrome address bar; this should open the right flag when loaded.
- Set the Hardware Media Key Handling flag to Disabled.
- Restart Google Chrome.