No keyboard on Bluetooth Pairing Request input

It recently happens frequently, that when I'm trying to pair a Bluetooth device (which asks for a passkey), the "Bluetooth Pairing Request" dialog pops up, but without a keyboard to type in the passkey.

When this happens, it happens with the native iOS connection manager (settings) and also 3rd party bluetooth apps. Only a restart of the iPhone helps to recover from it, in order to have the keyboard also popping up.

I'm not sure yet what exactly triggers the keyboard to go missing. As we're developing with our Bluetooth device, I'm pairing and unpairing quite frequently.