Do you mean preview in details page, or doclib prevew?
It seems this transformation has a source limit of 20 MB. You should set maxSourceSizeKBytes property for this transformer to a greater value. Unfortunately I don't know this property. It should be something like content.transformer.ffmpeg.thumbnail.extensions.mp4.jpg.maxSourceSizeKBytes=30000 or content.transformer.ffmpeg.extensions.mp4.jpg.maxSourceSizeKBytes=30000.
Also, try to turn on transformer debug setting to get more info about the problem:
log4j.logger.org.alfresco.repo.content.transform.TransformerDebug=DEBUG
And take a look at this page for more info on transformers:
https://hub.alfresco.com/t5/alfresco-content-services-hub/content-transformation-debug/ba-p/289284
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