The Entity Reference field has two display widgets: "Autocomplete" and "Autocomplete (Tags style)".
Both widgets allow users to add new taxonomy terms to a vocabulary as long as "Create referenced entities if they don't already exist" is enabled on the edit tab of the field. Terms cannot be added if that tick box is not ticked.
Since there doesn't seem to be any difference in the functionality of the two widgets "Autocompelete (Tags style)" could be removed.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
I think I ran into a related issue but on D9/10, described at https://www.drupal.org/project/drupal/issues/3369893 💬 Autocomplete (Tags style) widget not working with multi-value field Fixed . The "Autocomplete (Tags style)" widget seems to act/look just like the normal autocomplete widget, even if it is a multi-value field.