Where are the generated video thumbnails?

I’m uploading video with rtmedia_upload_media REST API, all videos are in mp4 format. After I have activated the Audio/Video encoding service Free Plan, and I have click on Regenerate Pending Thumbnails to regenerate thumbnails of pending videos. The regenerate has run on 15 videos for only 1 thumb for video according to my setting. At the end the encoding used panel then say that I’m using 44.57Mb (for 15 thumbnails only ???) and the thumbnails are not visible. No new jpg images are then created in wp-content/uploads/rtMedia/users.

Where are stored the generated video thumbnails? Thanks