When there is only one item in the field, and the label is hidden the item.attributes is not used. If we want to add attributes from hook_preprocess_HOOK() we need to handle both cases, add attributes if several items to all items from $variables['items'][$index]['attributes] or when there is only one item, we have to add class to $variables['attributes'].
File path : core/modules/system/templates/field.html.twig
Code line : 51
{% if label_hidden %}
{% if multiple %}
<div{{ attributes }}>
{% for item in items %}
<div{{ item.attributes }}>{{ item.content }}</div>
{% endfor %}
</div>
{% else %}
{% for item in items %}
<div{{ attributes }}>{{ item.content }}</div>
{% endfor %}
{% endif %}
{% else %}
Remove the {% else %} condition in the {% if label_hidden %} and keep the implementation from {% if multiple %}.
Needs work
11.0 🔥
system.module
The change is currently missing an automated test that fails when run with the original code, and succeeds when the bug has been fixed.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Makes sense. I can see the same in Claro, Olivero and Starterkit Theme. Would be good to get some test coverage.