[random test failure] LayoutBuilderBlocksTest::testBlockPlaceholder failing

So MR!9676 (https://git.drupalcode.org/issue/drupal-3477586/-/jobs/2891429) shows that this test fails ~26/1500 runs.

At first glance, I can think of two ways the placeholder is not replaced:

1. Setting the $block_content in \Drupal::state() goes wrong/causes a race condition
2. The "magic" in \Drupal\layout_builder\EventSubscriber\BlockComponentRenderArray::onBuildRender does something unexpected and we end up in the if ($is_content_empty && $is_placeholder_ready) where we set the placeholder again, with similar content as the original one.

Let's assert both.

So looking at all the test failures being:

Drupal\Tests\layout_builder\Functional\LayoutBuilderBlocksTest::testBlockPlaceholder
    Behat\Mink\Exception\ResponseTextException: The text "is_content_empty &&
    is_placeholder_ready Placeholder for the "The block label" block" appears
    in the text of this page, but it should not.

I think we can safely say it's #2.

$content = $block->build() in \Drupal\layout_builder\EventSubscriber\BlockComponentRenderArray::onBuildRender ends up in \Drupal\layout_builder\Plugin\Block\ExtraFieldBlock::build.

The only way I can see that returning empty would be if we land in this if-branch:
if (!isset($extra_fields['display'][$this->fieldName])).

Which in turn would mean \Drupal\Core\Entity\EntityFieldManager::getExtraFields would return an array without $extra_fields['display'][$this->fieldName], which would mean in \Drupal\Core\Entity\EntityFieldManager::getExtraFields, $this->extraFields[$entity_type_id][$bundle] would not be set.

This is where it ends for me, I really have no idea what's happening to cause the random failure.