Here's an update. Received an email from Marriott customer care saying-
"We see that a Customer Service file was opened and forwarded to the relevant team to have a look into this. Our colleagues will follow up with you as soon as they have fully investigated your issues with these Free Night Awards. Your patience during these challenging times is greatly appreciated."
Who knows how long it will be. In the meantime, I will keep checking online to see if anything changes. If anyone else receives the 5 free night certificates and is either successful or unsuccessful attaching them to a reservation, please let us know. It would be good to have another data point.