Have a look at the Plex Media Scanner Credits.log file. I also have (had - as of today) a candidate that is (was) being picked up every day. In my case the video file was sort of corrupt towards the end and Plex started its search for credits in that section of the file.
Run ffmpeg with the same arguments that are passed to the Plex Transcoder app and check the output.
May 25, 2023 01:29:18.280 [0x207315b40] DEBUG - [JobRunner] Job running: FFMPEG_EXTERNAL_LIBS='/Users/htpc/Library/Application\ Support/Plex\ Media\ Server/Codecs/8217c1c-4565-darwin-aarch64/' "/Applications/Plex Media Server.app/Contents/MacOS/Plex Transcoder" -codec:v hevc -y -threads 6 -skip_frame noref -ss 2735 -i "/Volumes/DROBO/Media/TV Shows/The Walking Dead/Season 06/The Walking Dead - S06E08 - Start to Finish.mp4" -vf "[in]fps=0.5,scale=w=320:h=320:force_original_aspect_ratio=increase,showinfo[out]" -vsync passthrough -f image2 "thumb-%05d.jpeg"
May 25, 2023 01:29:18.280 [0x207315b40] DEBUG - [JobRunner] In directory: "/var/folders/yt/29d6r2b140gcdh9dnv_dh2m00000gp/T/PlexCreditsDetection-19e47723-50ab-4e4c-b6ab-922c1c806fbb"
May 25, 2023 01:29:18.281 [0x207315b40] DEBUG - [JobRunner] Jobs: Starting child process with pid 3413
May 25, 2023 01:29:18.429 [0x207315b40] ERROR - BufferingLineReader: failed to read line (error: -1)
