I think this is because in dark mode the body text is white and camouflages into the white background.
I modified the text to red using my browser's dev tools and was able to get the text to appear:

I haven't verified this, but I think the white text in the code blocks and rendered diagrams are unaffected by virtue of being in their own subcomponents.

I might be wrong, but it seems the handler takes a screenshot of the message CardBody.
I wonder if capturing the entire Card instead would fix this issue (so the text would be visible in its respective background)