Upload speed capped at 1MB/s even when I set it unlimited

[sing]

My ISP upload speed is 30 Mbps. I'm using Windows 10 pro and qbittorent v.4.1.6. I have set my upload speed to unlimited, but it has always capped at 1 MB/s for some unknown reason. Every time  it hits 1.1MB or 1.2MB, the program quickly drop it back to 1MB/s.