By default, Single-Select drop-down fields have an arrow showing that there are values to select, but Multi-Select drop-down fields do not have the arrow. This code snippet addresses the issue.
.select2-container--default .select2-selection--multiple .select2-selection__rendered:before {
content: "\e5c5";
font-size: 24px;
color: #000;
font-family: Material Icons;
position: absolute;
right: -8px;
bottom: 5px;
}