Trouble with Synthesia, your keyboard, or adapter? Think you found a bug?
When describing problems, always mention your OS and game version (shown at the bottom of the title screen).
If your keyboard has USB or MIDI ports, there is a tremendously high chance (>99%) it will work with Synthesia. See what you'll need on the keyboards page.
I have an iPad connecting to a Yamaha Clavinova (CLP240, 2006 model) via an Apple Lightning to USB Camera Adapter. This has worked for years on an old iPad and still does. On my new iPad Air it connects fine at first but stops seeing piano keys being played, usually in less than a minute, but shows no error messages. The new iPad started at iOS 12.2.0 but the issue remains with 13.3.0.
My thinking is that maybe the new iPad doesn't allow sufficient output power for reliable operation. I'm considering two options:
In the past I've seen the "This accessory uses too much power" message pop up. And connecting it through a powered USB hub solved that problem. (I actually got seven(!) MIDI devices connected at once that way.) If you're not seeing the "too much power" message, I'd be curious if the hub would fix the problem you're seeing, too.
Just as a double-check to make sure it's not Synthesia's fault: is the app version the same on both iPads? The version should be listed at the lower-right corner of the title screen.