Actual issue is described in this round-route solution: https://discourse.webflow.com/t/how-to-reformat-text-from-grist-table-to-webflow-rich-text-field/266264

In brief: direct copy paste of text from external source to Webflow CMS Rich text field often works ok - but it does not work via API created CMS Rich text item unless the text formatting is already correctly HTML.

It would be good if the formatting conversion would be available for the API actions too.