Yes, unfortunately the selection code won't work for multiple selections. You can update it manually to fix the issue. You would have to update:
<?php if (!(strcmp("Willing to Help as needed", (ValidatedField("volunteer_254","interest1[]"))))) {echo "selected=\"selected\"";} ?>
to:
<?php if (in_array("Willing to Help as needed",ValidatedField("volunteer_254","interest1"))) {echo "selected=\"selected\"";} ?>
Unfortunately each line will have to be updated manually since the value is static and not from a database.