Thumbnails not in Media Library

@terrainosaur

Did you try this
https://www.ads-software.com/plugins/regenerate-thumbnails/

Also, the thumnbail filename should always follow the same pattern based on the original filename.
https://path-to-original-file/filename.jpg
will be the settings used in your media library for image size, like :
https://path-to-original-file/filename-300×300.jpg,
https://path-to-original-file/filename-150×150.jpg
etc

Lastly, you could write your own function using this which is native to wordpress :
https://developer.www.ads-software.com/reference/functions/wp_get_additional_image_sizes/

Ran Regen. Thumbnails still not in Media Library.