Mosaic doesn’t support adding rich text fields into repeating subsections. We strongly advise against adding rich text fields to these subsections. Instead, use standard text fields to avoid data loss.
If rich text fields have already been used in repeating subsections, update the form template to replace these with standard text fields. Then, use the form refresh button to update the form to the active version.
Note that the button only appears if the System Property enableUpdateActivitiesButton is set to true, you can find more information here Refresh form template button