Contact form captcha not solvable if type is changed

If a contact form is submitted and found unsure, the initially shown captcha can be correctly solved. If the captcha is however changed to audio (or back to image afterwards), the captcha cannot be solved. On the backend I see that a different session ID is used for these alternative captchas. After a quick look at the module code (which might be a wrong assessment), I see that the mollom.js uses "edit-session-id", but this cannot be found in the HTML which is generated (I do however see a "edit-mollom-id" form field).