The user field of type select_advanced displays user names correctly in the dropdown menu. However, once a name is selected, the closed control shows “[object Object]”. The stored value is correctly, and the front end renders properly; the issue is display-only.
In the file inc/fields/user.php (around line 104), the code forces show_thumbnail to be true for every select_advanced user field. This enables the setObjectThumbnailTemplate() function in js/select-advanced.js, which assigns options.templateSelection to options.templateResult. The templateResult function returns a jQuery object. Although the bundled select2 handles a jQuery return from templateResult correctly, it displays “[object Object]” when using templateSelection.
This was confirmed in the browser console on the edit screen:
$sel.select2({ templateSelection: d => $('<span>').text(d.text) }); // shows [object Object]
$sel.select2({ templateSelection: d => d.text }); // renders correctly
.
