I had a similar issue (or the same?) when I was testing the forum module with different users with different permissions. I created the users in the CMS, logged in to the forum under their names to edit the profiles, saved them and the passwords were set to NULL. I rectified by manually copying in known passwords to the Member and MemberPassword tables (Password and Salt fields).

I put it down to the ridiculous behaviour of Firefox, which copies your user name (email) and password into appropriate fields if you have the Remember Passwords function set. It only copies the password into the Password field of the profile form, not the Confirm Password field, but the form still saves okay. It should not, as the two fields have different contents, but it does.