Changing the text displayed in an Entity Reference Autocomplete
You need to implement your own selection plugin for this.

I thought I've already answered a similar question but can't find it anymore. 

Assuming the target is a node, extend from \Drupal\node\Plugin\EntityReferenceSelection\NodeSelection, otherwise from its parent or appropriate other child class and override \Drupal\Core\Entity\Plugin\EntityReferenceSelection\DefaultSelection::getReferenceableEntities().

Make that available under its own plugin id (not default:something, just something) and select it in the entity reference field settings.