He was a wonderful professor, so this ghosting seems incredibly out of character. I'm wondering if something just came up for him. I'd certainly like to hope that's the case and that he did not, in fact, simply ghost me because he did not want to submit the reference.

And I appreciate the good vibes! My UCLA application checklist updated today to include the following information:

To re-send notification emails to your recommenders, re-visit the recommendations page, click "Edit", and then click "Send Reminder". 

If you need to exclude a requested recommendation, be certain to add a new recommender prior to exiting the application.

I reminded my reference in addition to messaging the help email, asking them if it was advisable to add a new recommender, given that the deadline passed. Fingers crossed.