When loading an AU instrument (either in the midi track view's instrument popup or in the channelstrip dialog), its window is automatically opened. Sometimes pressing the X on the AUs window does not work, directly after the inital opening and without any app switching or track changing. On my side it happens about every 10th to 20th AU opening.
My observation/bug-report is an addentum to the reply Rim (WaveMaschineLabs) made on the Audiobus forum:
WaveMachineLabs wrote:Thanks, I'm aware of the GUI closing issue. It occurs after you suspend and resume the iPad. It's on my list to investigate today.