This might be related.
In Final Cut Pro X automation gets changed when it is played back (which makes it unusable). Only happens if the plugin editor is open. Probably providing unexpected feedback to the host.
Final Cut Pro X is kinda quirky when it comes to automation. If playback is stopped and an automated parameter gets changed in the plugin editor, the entire automation gets changed relatively (in the vertical axis), such that the automation curve matches the changed parameter at the playhead position.
Another AU issue that might be related: Set Logic Pro X to write automation data. Change a parameter in the plugin editor during playback. Automation does not get recorded at all. If I remember correctly this worked before the introduction of AudioProcessorParameter.
Final Cut Pro X Trial 10.2.3 (available at https://www.apple.com/final-cut-pro/trial/)
Logic Pro X 10.2.1