In Final Cut Pro X the automation curve gets unintentionally changed on playback when the plugin editor is open. JUCE is probably providing unexpected feedback to the host. This does not happen with other non-JUCE based plugins.
I’ve already mentioned this a few month ago, but I guess it slipped through because it was not in its own thread (AAX plugin, automation gets all messy after a while).
An issue with Final Cut Pro X automation was now also confimed by @anthony-nicholls (Are there any hosts (de)constructing plugins on different threads?).
Note: Final Cut Pro X is kinda special 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.
JUCE 4.2.2 (latest available development commit 7abbf67a),
Final Cut Pro X 10.2.3