Chapter thumbnails shown but are blank

Server Version#: 1.19.3.2793
Player Version#: 4.33.1

I have a Plex Pass and am using the DVR functionality. After each recording, I am using the POSTPROCESSING functionality to run a script that adds comskip chapters to the recorded video. Basically, the workflow is:

step 1: recording.ts → Plex Transcoder → working.mkv
step 2: recording.ts → Plex Commercial Skipper → comskip.ffmeta
step 3: working.mkv + comskip.ffmeta → remux → recording.mp4
step 4: delete recording.ts

The postprocessing is successful. recording.mp4 is added to the library and Plex recognizes that there are chapters in the video. However, most of the time, the chapter thumbnails are blank (no image preview).

Plex is configured to generate chapter thumbnails on library update AND as a scheduled task. However, neither event triggers thumbnail generation for chapters.

UPDATE: It appears that the scheduled task generates thumbnails, just not the library update setting. Have to wait up to 24 hours for the thumbnails.

The final re-muxing command is:

/usr/lib/plexmediaserver/Plex\ Transcoder -y -hide_banner \
    -i "$WORKINGFILE" -i "$COMSKIP_CHAPTERS" \
    -c:v copy -c:a copy -c:s copy -c:d copy -c:t copy \
   "$NEWFILE"

Attached is portion of the log for a successful recording and post processing run.
log_sample.txt (132.9 KB)

The chapters are present, but no chapter thumbnails:

Are there known bugs related to chapter thumbnails in post-processed TV shows?

Tested both web player (Chrome, Firefox) and Roku player. All have the issue.

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