The htmlview component does not behave correctly in Windows Mobile. On the first request for drawing, the input text which has been passed so far is displayed (that's OK) and the input is closed (that's bad). This means that all other DTM_ADDTEXTW messages are simply ignored.
I have reported this to Microsoft. Microsoft considered it as a bug, but will not fix it unless manufactures of the devices will require it.
This problem is not present in PocketPC 2003 and 2002.