The issue is only present on WordPress.com in the Editor view. With Gutenberg plugin v.8.3 at https://jurassic.ninja/ the font is in the editor is the same as in the Preview or Published view.
The font in Editor view to be the same as in Published view
The font in Editor view is different from Published view. It's just the same font as in the Paragraph Block, instead, it should be similar to the Preformatted Block font.
WordPress.com Editor view vs Published page view

WordPress.com Editor vs Gutenberg plugin v.8.3 at https://jurassic.ninja/

Confirmed for Gutenberg v8.3:
This is an issue for Varia themes. It does not exist with a different theme, like Seedlet. So the issue is probably caused by Varia editor CSS.
This should be fixed now. Let me know if you still see issues @zdenys
Most helpful comment
This should be fixed now. Let me know if you still see issues @zdenys