Hi,
I have the similar problem with my smart keyboard and iPad Pro, especially since I updated to iOS 12.
I also get the Accessory Not Supported popup, but just a few times. But what I do get is the "device connected sound", same sound as when you connect your lightning cable. But the thing is, nothing is connecting or disconnecting when the sound is heard. So I'm guessing it is the keyboard disconnecting? Very strange the first times I heard it...
I figured out today that the keyboard works if I have the lightning cable connected and charging the iPad. I disconnected it and it worked fine until around 85-90% battery power, then the "device disconnected" sound was heard again... very annoying!
Anyone else experiencing this?