\Drupal\Core\Entity\Entity\EntityFormDisplay::buildForm and \Drupal\Core\Entity\Entity\EntityFormDisplay::processForm both assume that a field has a weight set, it shouldn't.
This patch sets the weight to 0 if one is not set.
Closed: duplicate
9.4
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
For anyone looking to suppress this warning until the related issue lands, this is the patch that's working for me. Similar to the one posted except it needs the null coalescing operator:
$element[$name]['#weight'] = $options['weight'] ?? 0