We use something called foreign objects in SVG to allow complex, HTML labels on shapes. All modern browsers including Chrome, Firefox, Safari and Microsoft Edge support this (Internet Explorer 11 and earlier does not). In draw.io on IE 11- we can account for this dynamically, but exported SVG will not show all labels when the SVG is viewed on IE, most SVG editors (such as Inkscape and Illustrator) and some online services (such as Wordpress) and will show [Not supported by viewer] instead (or will have a parse error in the case of WordPress).
We are aware of the problem and have spent a considerable amount of time researching solutions. However, the subject is very complex and we don't have a fix so far. We will continue to investigate it.
In the meantime, you might consider exporting to a PDF, this is also a vector format, or disable formatted text and word wrapping for all labels in the diagram before exporting as SVG as follows:
- Select all elements of the diagram (right click, Select All) and uncheck the Formatted Text option.
- Select all vertices of the diagram (right click, Select Vertices) and uncheck the Word Wrap option.
Note that with formatting disabled, it is still possible to apply formatting to the label, just not to parts of the label.