Flow's wikitext area doesn't respect the text formatting user option/preference (monospace, serif, sans-serif, etc.).
Top of the "Editing" section in https://www.mediawiki.org/wiki/Special:Preferences#mw-prefsection-editing
name = editfont - One of the MediaWiki DefaultUserOptions https://www.mediawiki.org/wiki/Manual:$wgDefaultUserOptions
See T72028: Should Mediawiki UI change font-family for textareas?