I'm running two ipad air2, both on IOS 12.3.1. I set the first to broadcast the screen, both WiFi and Bluetooth are ON. Set the other to receive. The "slave" will stay on the "select device" screen fir what seems like forever UNTIL, on the master, i back up to the settings screen. At that point, the master shows up on the slave's "select device" window. Everything else proceeds normally. I can repeat this over and over.
I also get this behavior when I use an iPhone 8 as the slave.
Is that "normal" behavior?
On iOS, the master will stop broadcasting if it hasn't been connected to in a while. In that case, you can click the device linking button in the top toolbar on the master and click Refresh Broadcasting. Does that fix it?