mdb-select into mdb-accordion does not show properly

Code:

 ... <div class="row mb-2">
    <div class="col-md-12">
      <mdb-accordion [multiple]="false" >
        <mdb-accordion-item [collapsed]="false" [customClass]="'z-depth-1'">
          <mdb-accordion-item-head  [customClass]="'grey lighten-4 border-0'">Filters</mdb-accordion-item-head>
          <mdb-accordion-item-body (animationStateChange)="onAnimationStateChange($event)">

            <div class="row">
              <div class="col-md-6">
                <mdb-select [multiple]="true" [filterEnabled]="true" [options]="optionsSelect" placeholder="Choose your option"></mdb-select>
              </div>
            </div>.............