I am experiencing this behaviour on a new site with Drupal 10.4.1.
Typing in the value field performs the autocomplete search, but clicking on an option puts the URL in the value field. The Definition URI field remains blank.
I have only experienced this on the node edit page, and it only occurs until I add a second instance of the field (for a multi-valued field).
Then both the original and the added fields behave properly.
It appears that the included javascript (subject_autocomplete.js) is not run when the page is initially loaded. Once I click the "Add another item" button it then runs for the original and new fields.
I have tried with both the Claro and Olivero as the admin field, it is the same for both. I'm trying to figure out why it doesn't run at the initial page load.
I am not seeing this behaviour with Drupal 10.