If this does not work...
broken_fapi_autocomplete.php
$form['nodes'] = [
'#title' => $this->t('Select articles'),
'#type' => 'entity_autocomplete',
'#target_type' => 'node',
'#tags' => TRUE,
'#selection_handler' => 'default',
'#selection_settings' => ['target_bundles' => ['article']],
'#default_value' => $default_nodes,
];
...try adding an increased maxlength value:
fixed_fapi_autocomplete.php
$form['nodes'] = [
'#title' => $this->t('Select articles'),
'#type' => 'entity_autocomplete',
'#target_type' => 'node',
'#maxlength' => 1024,
'#tags' => TRUE,
'#selection_handler' => 'default',
'#selection_settings' => ['target_bundles' => ['article']],
'#default_value' => $default_nodes,
];