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I tried adding the #ajax property to an autocomplete entity reference field on a Content Type creation form but I don't seem to be getting the callback when the field value is changed. I tried adding #ajax to some other simple fields on the same page and the callback worked, so it's specific to the entity reference field. (The reason I'm doing this is to catch and handle the situations where there are zero or > 1 matching references and put up a little form to allow the user to add the missing entity or choose between the multiple matching ones).