Yahoo Mail recipient autocomplete / contact search has stopped working.

This feature used to work normally, but recently stopped.

Symptoms:
• In Compose → To, typing part of an email address produces no autocomplete suggestions.
• Clicking the To field opens the contact picker, but it is completely empty.
• I have existing Yahoo Contacts, and I can open individual contacts and see their email addresses.
• Searching my Contacts produces no results, even for contacts that are visibly present in the contact list.
• I tested an email address that I have previously sent mail to, but it still does not appear as an autocomplete suggestion.
• I created a brand-new test contact successfully, but it also does not appear in the To-field autocomplete.
• I can manually enter the complete email address and press ENTER; Yahoo then accepts it as a recipient.
• Enhanced Contacts is enabled.
• The problem occurs in both Microsoft Edge and Google Chrome.

Please investigate the Yahoo Contacts / recipient autocomplete service or indexing associated with my account. The contacts themselves are present, but the Contacts search and Compose recipient suggestions appear unable to retrieve them.

This appears to be an account/server-side Yahoo Mail issue rather than a browser issue.