Japanese not shown correctly in preview image

This is a preview (from Twitter) for a cast I just posted. The squares on the left should all be Japanese.

szd-WOyU

The cast:

https://asciinema.org/a/3G322ywudzGgKAvTUtYfBEYj0

The cast itself shows Japanese just fine, I guess whatever is generating the thumbnail image doesn’t have the right fonts installed. Any chance of getting this fixed?