Custom domain email address not available in iCloud mail

Same issue as Custom domain e-mail not available in iCl… - Apple Community, but not resolved by the steps provided.

I have two custom domains, and have set up email addresses with each of them. They've been working fine for the past year or so. However, I seem to be unable to add a new email address now.

I've tried deleting, waiting, and re-adding the email address to no avail. So even though under my iCloud plus settings I can see the new email address under Custom Domains > Manage email addresses, the newly created (and re-created, multiple times) email address does not appear in iCloud Mail prefs > Account > Custom Domain Email Addresses.

I initially tried setting this up on my iPhone, then again on web (macOS, Safari).