Locked to device, not showing up on that or any other.

@robfenton if the controller gets stuck with a solid cyan LED and ignores the standard pinhole reset, the internal firmware is likely caught in a frozen state or a "ghost" connection loop where it thinks it is still actively communicating with the tablet. You could try and leave the controller sitting until the battery completely dies. Once it is completely dead, plug it into a charger for 10 minutes and turn it on. When you power it back on, it should boot normally with a flashing pairing light, allowing you to connect it as a new device.

Even if your tablet says it is unpaired, its Bluetooth radio might still be pinging the controller. Turn Bluetooth completely off in the Xiaomi tablet's settings as well as any other nearby device that uses Bluetooth. Once they are all off, press the reset button again. The cyan light should transition from a solid glow to a rapidly flashing light, meaning it has successfully realized it is disconnected and is searching for a host. You can also try forcing it to boot into a different operating mode. Press and hold R2 + Power (or A + Power) simultaneously for about 5 to 8 seconds to force it into standard Android or Switch mode. The LED should change from cyan to a flashing red, green, or blue light, indicating it has overridden the frozen state and entered a new pairing mode.

Let us know what you find out.