If custom field is empty, use one from a previous post

if(!get_field('afbeelding_onder'))
{
    $prev_post = get_previous_post();
    echo '<p>' . get_field('afbeelding_onder', $prev_post->ID) . '</p>';
}
else
{
    echo '<p>' . get_field('afbeelding_onder') . '</p>';
}

not tested. you might want to include a while loop, if the previous post does also not have that value set?

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