Keep custom settings on the block

js/gallery-block.asset.php

@@ -1 +1 @@
-<?php return array('dependencies' => array('react', 'react-dom', 'wp-api-fetch', 'wp-block-editor', 'wp-blocks', 'wp-components', 'wp-components/build-style/style.css', 'wp-data', 'wp-element', 'wp-i18n', 'wp-polyfill'), 'version' => '1eb11fc0b636111ccc4876191f5e87c7');
+<?php return array('dependencies' => array('react', 'react-dom', 'wp-api-fetch', 'wp-block-editor', 'wp-blocks', 'wp-components', 'wp-components/build-style/style.css', 'wp-data', 'wp-element', 'wp-i18n', 'wp-polyfill'), 'version' => '95635dcdce3468c638dab46e572af7a3');

js/src/gallery-block/edit.js

@@ -41,6 +41,11 @@ const Edit = ( { setAttributes, attributes, className, isSelected } ) => {
 	const [ loading, setLoading ] = useState( false );
 
 	const preparedAttributes = useMemo( () => {
+		// Do not override block settings with defaults on existing ones.
+		if ( 0 !== attributes.selectedImages.length ) {
+			return attributes;
+		}
+
 		const defaultAttrs = {};
 
 		// eslint-disable-next-line no-unused-vars