How to (force) recreate chapter images for specific movie/show/season?

[TheDreadPirate]

Am I able to extract chapter images only, without replacing all of the existing metadata?
I have chapter image extraction to run at the default scheduled time & on library scan, but I only enabled this feature after my library was populated.
Chapter images are only extracted for newly added files, not existing files.
Manually running the scheduled task does not generate chapter images on the old files. When manually run, it takes less than 5 minutes.
The only way I can generate chapter images for older files is to replace all metadata, which deletes/replaces the poster & fanart images scraped by TMM (which I prefer).

I setup a test library to replicate your situation: Already have a populated library, but didn't enable trickplay/chapter image extraction when creating the library.

After the initial scan finished on the two movies in the test library, I went back and enabled both trickplay and chapter image extraction.  Running the scheduled task for both triggered it to go back and extract chapter images and generate trickplays.

Before I ran the job, this row in the UI didn't exist.

   

What version of Jellyfin are you running?  Can you share your log via pastebin?  Maybe something else is going on with your server to cause this.