The insertRow method creates a row that replicates an existing row; it's a design choice, in order to avoid requiring a lot of parameters specifying the configuration of the new row.
The text content of a cell is hosted in one or more paragraphs. The cellValue (or setText, when applied to a table cell) method updates the first paragraph of the target cell. The subsequent paragraphs, if any, remain unchanged. Note that this feature allows the user to individually select/update/delete a particular paragraph in a cell.
As a consequence, all the embedded paragraphs must be explicitly removed in order to clear a cell as soon as it's suspected to be a multi-paragraph one.
We can use getCellParagraphs in order to get the list, then delete each paragraph using removeElement. Example: