Have a phone rep help you cancel. Then ask the rep if they can see that your cert and points are back. (I know from experience that phone reps see my points balance adjusting well before I can see that on the Bonvoy website.)
I've seen anecdotal reports of certificates not reappearing in your account when a reservation is canceled.
Marriott IT is awful. Yesterday I tried to make a 30K points reservation. I had one 35K certificate in my account, but the Bonvoy website did not offer me the option to apply my cert. (I was using Microsoft Edge. I know that Brave sometimes has problems with the Marriott website.) I had to phone. The phone rep got it done for me (on her second attempt).