Why is my mouse losing battery when it’s constantly plugged in? (Speed light)

Hey. I recently got this awesome mouse and I like it a lot however have noticed that upon turning my pc on after not being on it for a day, the green (battery charge) dot at the top flashes as if it’s charging and is only 2/3 charged.

The mouse can do Bluetooth but I don’t have the dongle so I just use it plugged in constantly, just curious why it would lose charge.

Thanks.