I noticed this problem when I added the client property to suppress htmlarea for testing with Selenium.
It just sets htmparea_p = 0 in the richtext widget tcl procedure right after it checks the parameter. Anyway if you read the code you can see that it can't possibly work, since it checks for the "value" of the widget not the mode.
The test I wrote is using the forms in the assessment package to add a new question.
