Imovie 10.1.9 clips turn solid black in Final Cut Pro X 10.3.4 after "Send Movie To Final Cut Pro"
Sorry for panic. Answer has been found.

Problem: Final Cut Pro X was unable to see 4k 60fps videos captured by iOS devices.

Reason: Unknown.

Solution: Need to update Final Cut Pro X to 10.4.1 version.

Source: https://support.apple.com/en-us/HT208131