Meta Box

Did some further research and seems to be a plugin conflict with The Events Calendar.

Disabled that last one to investigate further on a local setup.

When I add a "normal" select, it is saved in the block:

 [
                'name'            => 'Select Advanced',
                'id'              => 'select_advanced',
                'type'            => 'select_advanced',
                // Array of 'value' => 'Label' pairs
                'options'         => array(
                    'java'       => 'Java',
                    'javascript' => 'JavaScript',
                    'php'        => 'PHP',
                    'csharp'     => 'C#',
                    'objectivec' => 'Objective-C',
                    'kotlin'     => 'Kotlin',
                    'swift'      => 'Swift',
                ),
                // Allow to select multiple value?
                'multiple'        => false,
                // Placeholder text
                'placeholder'     => 'Select an Item',

            ],

Saved block:
<!-- wp:meta-box/wedstrijdkalender-inhoud {"id":"mb-block-bf398a9f-2c92-4b22-b00a-6678cac348ca","data":{"hans":"TESTJE","select_advanced":"csharp"}} /-->