It can be seen in the database table moodle_question, the questiontextformat are always set to 0 after importing. In the xml file:
Replace the "html" by "moodle_auto_format", "plain_text", "markdown", All of them give the same result questiontextformat=0 in the database table. A simple xml file is attached, just try the Moodle XML import of this file.
A little testing on a new install of moodle 1.9.9 shows that it affect all build in question types as well as plug-in question. However, it seems that there is no problem for the import to a moodle 2.0.