Phone: Samsung Galaxy S7 Edge (US)
Android: 6.0.1 Marshmallow
Video of problem: https://www.dropbox.com/s/o6bhe8nbd4q7xze/20170327_012752.mp4?dl=0
Attempting to undock Swiftkey sometimes fails in landscape mode when the app has opened it in fullscreen mode. Clearing data or reinstalling doesn't fix it, but rebooting does. This suggests to me that it's something outside Swiftkey interfering with it undocking.
It always undocks successfully in portrait mode, and I don't think I've seen it fail in landscape apps that don't usually open it as a fullscreen keyboard.