I emailed Marriott Rewards about the situation I spelled out above and here is their response:

Dear c699534:

Thank you for contacting Marriott Rewards.

I apologize that reservations made prior to our system upgrade will not show on line that the certificate is attached to the reservation.

Our system does show that your 083 award good for 7 nights in a hotel category 1 - 5 award is attached to your -XXXXXXXX confirmation number.

Thank you for choosing Marriott.
Sincerely,
Connie McRae
Marriott Rewards
Correspondence Specialist

So I guess I'm good to go, regardless of what the website says. Would it be prudent to contact the hotel directly to confirm they have my reservation in their system correctly as well? I'd plan on doing it via email as well to ensure a paper trail.

Appreciate the advice Mr. Vker on the email route. Paperwork in hand is definitely more reassuring than a "It's all OK" over the phone!