Thanks for you help. If I double click the mouse on a column, then choose Insert HTML, a HTML Editor popup window is shown and at the bottom of the Editor it displays ‘POWERED BY TINY’.
If I enter some text in the HTML editor, then click Insert/Edit link, an Insert Link popup is displayed. Once I enter the Url value, if you click OK, then click the Source Code icon, you can see that the Url value is always converted to a relative URL (../news/ instead of https://websiteurl.com/news/).
This does not happen when I am using the regular HTML editor, only when adding HTML to a wpDataTables table/column.
Please let me know if you can reproduce the problem otherwise I can send screenshots/video.