You are right. When I switched to the default Cassiopeia template it did show up. I was able to make it appear again by adding the following to the override of the default CB template:
Code:
.cb_template.select2-container--bootstrap .select2-dropdown .select2-results .select2-results__options:not(.select2-results__options--nested) {
max-height: 350px;
overflow-x: visible;
overflow-y: clip;
}