This code snippet can fix a dropdown functionality in Ultimate Member if it is broken due to plugins conflict.

re-initialize_dropdown_fields.js

/**
 * Re-initialize dropdown functionality.
 * Add this code to your custom JS file.
 */
jQuery( function () {

	/**
	 * Verifies that there is no empty value.
	 */
	function unselectEmptyOption( e ) {
		var $element = jQuery( e.currentTarget );
		var $selected = $element.find( ':selected' );

		if ( $selected.length > 1 ) {
			$selected.each( function ( i, option ) {
				if ( option.value === '' ) {
					option.selected = false;
					$element.trigger( 'change' );
				}
			} );
		}
	};

	/**
	 * Re-initialize dropdown functionality.
	 */
	function reInitializeDropdownFields() {
		if ( 'function' === typeof jQuery.fn.select2 ) {

			// Remove old dropdowns.
			jQuery( '.um-s1, .um-s2, .um-s3', '.um-form' ).filter('.select2-hidden-accessible').select2('destroy').off('select2:select').siblings('span.select2').remove();

			// Initialize new dropdowns.
			jQuery( '.um-s1' ).each( function ( e ) {
				var obj = jQuery( this );
				obj.select2( {
					allowClear: true,
					dropdownParent: obj.parent()
				} ).on( 'change', unselectEmptyOption );
			} );

			jQuery( '.um-s2' ).each( function ( e ) {
				var obj = jQuery( this );
				if ( obj.parents( '.um-custom-shortcode-tab' ).length ) {
					var atts = {
						allowClear: false
					};
				} else {
					var atts = {
						allowClear: false,
						minimumResultsForSearch: 10,
						dropdownParent: obj.parent()
					};
				}
				obj.select2( atts ).on( 'change', unselectEmptyOption );
			} );

			jQuery( '.um-s3' ).each( function ( e ) {
				var obj = jQuery( this );
				obj.select2( {
					allowClear: false,
					minimumResultsForSearch: -1,
					dropdownParent: obj.parent()
				} ).on( 'change', unselectEmptyOption );
			} );
		}
	}

	/**
	 * Re-initializes dropdown functionality on the page load with timeout.
	 * Advice: Try to enlarge a timeout if the issue is not solved.
	 */
	setTimeout( reInitializeDropdownFields, 99 );

	/**
	 * Re-initializes dropdown functionality on the Elementor popup open.
	 * This part in needed for the Elementor popup only.
	 */
	jQuery( document ).on( 'elementor/popup/show', function () {
		jQuery( window ).trigger( 'resize' );
		reInitializeDropdownFields();
	} );

} );

This gist is a part of the article Fix dropdown field functionality

Documentation: https://docs.ultimatemember.com/
Support forum: https://wordpress.org/support/plugin/ultimate-member/