In the uc_cart_preprocess_block() there is a code:
if ($variables['block']->module == 'uc_cart' && $variables['block']->delta == 0 && $variables['block']->subject) {
...
}
Code is checking for the $variables['block']->delta and comparing with zero. However the correct delta for the uc_cart block is cart, see:
$blocks['cart'] = array(
'info' => t('Shopping cart'),
'cache' => DRUPAL_NO_CACHE,
'pages' => 'admin*',
);
The condition evaluates to TRUE on PHP 7.x and lower, but on PHP 8, the comparission with zero is FALSE and therefore the block is not themed correctly.
Enable uc_cart block on PHP 7.x and PHP 8.1 and compare its HTML markup.
Fix the delta to the correct value.
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