I recommend you to email marriott reward desk to clarify this. As the answer to me was "negative", that means bonus nights are not counted towards the limited time challenge.

The reply from them to my similar request is:

"Thank you for your reply.

Only nights your personal stay that are paid directly to the hotel will qualify toward this offer. Promotional nights cannot be included.

Thank you for choosing Marriott."

My question was:
Since I have signed up for the double night bonus, does it mean that the bonus nights will be counted towards these 18 nights?