Rich text in different systems can mean completely different things. Webflow supports basic HTML for formatting as rich text content. If the source of your feed into the API is using something not matching what Webflow expects you can get different/unpredictable results.
It is possible with some tools (automation) to normalize content before pushing it into another system. I have not tried to do what you are attempting but that is where I would start.
You might want to look at Zapier Formatter: Automatically format text the way you want | Zapier