Currently (2021-10-28), Custom HTML embedded in a Rich Text field (via the Designer) is lost, when retrieved via the API. (Same thing happens with CSV export.) All we see is a plain
<div></div> with no indication that it was ever a custom HTML segment.
I can understand filtering out ill-formed and/or unrecognized HTML on import, as a safety feature. But this is our own content, added through Webflow's development environment. We should be able to get it back out again, just as it was put in.
Even if it takes a different API call to get/set the custom bits, and to know where they belong, it would be better than losing them entirely.