TYPO3 Core

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at http://review.typo3.org/44061

A similiar usecase where I get this is having such a relation

        'categories' => array(
            'exclude' => 1,
            'l10n_mode' => 'exclude',
            'l10n_display' => 'defaultAsReadonly',
            'label' => $ll . 'tx_news_domain_model_news.categories',
            'config' => array(
                'type' => 'select',
                'renderMode' => 'tree',
                'treeConfig' => array(
                    'dataProvider' => \GeorgRinger\News\TreeProvider\DatabaseTreeDataProvider::class,
                    'parentField' => 'parent',
                    'appearance' => array(
                        'showHeader' => TRUE,
                        'allowRecursiveMode' => TRUE,
                        'expandAll' => TRUE,
                        'maxLevels' => 99,
                    ),
                ),
                'MM' => 'sys_category_record_mm',
                'MM_match_fields' => array(
                    'fieldname' => 'categories',
                    'tablenames' => 'tx_news_domain_model_news',
                ),
                'MM_opposite_field' => 'items',
                'foreign_table' => 'sys_category',
                'foreign_table_where' => ' AND (sys_category.sys_language_uid = 0 OR sys_category.l10n_parent = 0) ORDER BY sys_category.sorting',
                'size' => 10,
                'autoSizeMax' => 20,
                'minitems' => 0,
                'maxitems' => 99,
            )
        ),

Important is the

'l10n_display' => 'defaultAsReadonly',

The fix would be the same in different class: sysext/backend/Classes/Form/Element/SelectTreeElement.php:49

$selectedNodes = $parameterArray['itemFormElValue'] ?: [];