Is it compatible with Bootstrap Barrio?

Hi Chris. Sure!

{#
/**
 * @file
 * Default theme implementation to display a block.
 *
 * Available variables:
 * - plugin_id: The ID of the block implementation.
 * - label: The configured label of the block if visible.
 * - configuration: A list of the block's configuration values.
 *   - label: The configured label for the block.
 *   - label_display: The display settings for the label.
 *   - provider: The module or other provider that provided this block plugin.
 *   - Block plugin specific settings will also be stored here.
 * - content: The content of this block.
 * - attributes: array of HTML attributes populated by modules, intended to
 *   be added to the main container tag of this template.
 *   - id: A valid HTML ID and guaranteed unique.
 * - title_attributes: Same as attributes, except applied to the main title
 *   tag that appears in the template.
 * - content_attributes: Same as attributes, except applied to the main content
 *   tag that appears in the template.
 * - title_prefix: Additional output populated by modules, intended to be
 *   displayed in front of the main title tag that appears in the template.
 * - title_suffix: Additional output populated by modules, intended to be
 *   displayed after the main title tag that appears in the template.
 *
 * @see template_preprocess_block()
 *
 * @ingroup themeable
 */
#}
{%
  set classes = [
    'block',
    'block-' ~ configuration.provider|clean_class,
    'block-' ~ plugin_id|clean_class,
  ]
%}
<div{{ attributes.addClass(classes) }}>
  {{ title_prefix }}
  {% if label %}
    <h2{{ title_attributes }}>{{ label }}</h2>
  {% endif %}
  {{ title_suffix }}
  {% block content %}
    <div{{ content_attributes.addClass('content') }}>
      {{ content }}
    </div>
  {% endblock %}
</div>

1. Doesn't work, because the attributes are not written anywhere: "Refresh button" does a few moments of loading and that's it. If I close the attribute configuring sidebar then, and open it again without leaving Layout Builder page, the attributes are gone.
2. Looks exactly the same on Bartik.

Chris, I got some new intel here.
Today I tried to use LBCA on user profile page layout. And, for some unexplainable reason, it worked like a charm.
So it does work on user pages, but it doesn't work on regular articles, although this theme's got a copycat of a block.html.twig file from Bartik. Weird!

Hi Chris. In my instance alert is on top of the page, and I can clearly see the "Update" button and press it.
Although I gotta tell you that now, a few Drupal updates later, it magically works as intended. I have no idea what exactly fixed it, but it's fixed. ¯\_(ツ)_/¯
As I seem to be the only one who had this problem, I guess the issue can be closed now. Thank you for trying to solve this.