Apparently, all the formats are deleted and a new span is created with the last format selected

 

Exemple:

with this code

 

<span><portal:html style="FONT-FAMILY: Wingdings" name="test">test</portal:html></span>
<span style="COLOR: red">test2</span>

Select all the text and select another color (ie. orange)

 

New code:

 

<span style="COLOR: #ffcc00"><portal:html name="test">test</portal:html>test2</span>