Угловой генератор событий

Решено! Я добавил onChange, как предложил @webdevius:

В HTML моей панели добавлен по изменению

<div class = "filter-multiselect">
  <p-multiSelect [options] = "items"
                 [(ngModel)] = "itemsSelected"
                 styleClass = "multiSelect"
                 [defaultLabel] = "labelComponent"
                 (onPanelShow) = "handlePanelShow()"
                 (onPanelHide) = "handlePanelHide()"
                 (onChange) = "filtersApply()"
                 panelStyleClass = "panelStyleClassCustom"
                 [ngClass] = "isOpen ? 'multi-select-open' : '' "
    [maxSelectedLabels] = "0">
    <ng-template let-element let-i = "index" pTemplate = "item">
      <div class = "ui-multiselect-item-text">{{element.label}}</div>
    </ng-template>
  </p-multiSelect>
</div>

и излучать при выборе фильтра (теперь не зависит открыта панель или нет)

import { Component, OnInit, ViewEncapsulation, Input, ElementRef, EventEmitter, Output } from '@angular/core';
import { MultiselectBaseModel } from '../app-models/MultiselectBaseModel';

@Component({
  selector: 'app-filter-multiselect',
  templateUrl: './filter-multiselect.component.html',
  styleUrls: ['./filter-multiselect.component.less'],
  encapsulation: ViewEncapsulation.None
})
export class FilterMultiselectComponent implements OnInit {
  @Input() labelComponent: string;
  @Input() items: MultiselectBaseModel[];
  @Input() itemsSelected: MultiselectBaseModel[];
  @Output() closeMultiselectEventHandler: EventEmitter<any> = new EventEmitter();

  isOpen = false;

  constructor(public el: ElementRef) { }

  ngOnInit() {}

  handlePanelShow() {
    this.isOpen = true;
  }

  handlePanelHide() {
    this.isOpen = false;
  }

  filtersApply() {
    this.closeMultiselectEventHandler.emit(this.itemsSelected);
  }

  handleClickMultiSelect(event) {

  }
}