A lot of Sony FX9 users have been experiencing problems with the latest version of FCPX 10.4.9, macOS Catalina 10.15.6, and Pro Video Codecs 2.1.2.

Source: https://www.newsshooter.com/2020/09/06/apples-response-to-sony-fx9-media-issues-with-final-cut-pro-x/