TYPO3 Core

The problem is located in FormEngineValidation.js at line 52:

        // Initialize input fields
        FormEngineValidation.initializeInputFields().promise().done(function () {
            // Bind to field changes
            $(document).on('change', FormEngineValidation.rulesSelector, function() {
                // we need to wait, because the update of the select fields needs some time
                window.setTimeout(function() {
                    FormEngineValidation.validate();
                }, 500);
                var $paletteField = $(this).closest('.t3js-formengine-palette-field');
                $paletteField.addClass('has-change');
            });

The setTiemout call calls the validate method which result in "de-select" the selected entries.

resolved in the meantime