Symptoms

When you export a QuickTime movie from Final Cut Pro and then import into back into Final Cut Pro, the resulting movie's Alpha Type is set to Straight by default when you export by choosing File > Export > Using QuickTime Conversion, and you export to ProRes 4444 with an Alpha Channel (Millions of Colors +).

Note: To check the Alpha Type of a clip in Final Cut Pro, select the clip in the Browser and choose Modify > Alpha Type.