Bug description:

When typeahead shows autocomplete list and an item in the list is active aria-activedescendant is not set on the input component.

Plunker/StackBlitz that reproduces the issue:
No special setup is needed, it can be observed for any example right on the documentation website https://valor-software.com/ngx-bootstrap/#/components/typeahead?tab=overview. Do inspect on the input, type something to trigger autocomplete list and observe that aria-activedescendant is not added on the input as expected.

<input class="form-control ng-valid ng-dirty ng-touched" aria-expanded="true" aria-autocomplete="list" aria-owns="ngb-typeahead-3">

Expected behavior
When item in the autocomplete list is active then input component on which the typeahead directive is used has aria-activedescendant attribute set with the value of the attribute being the id of the active item:

<input class="form-control ng-valid ng-dirty ng-touched" aria-expanded="true" aria-autocomplete="list" aria-activedescendant="ngb-typeahead-3-3" aria-owns="ngb-typeahead-3">

Upon investigation of the code, found the following:

1. typeahead directive sets the attribute when activeChangeEvent is fired from the list container
   this._container.activeChangeEvent.subscribe((activeId: string) => { this.activeDescendant = activeId; this.changeDetection.markForCheck(); });
2. The typeahead container fires the event on the active setter:

set active(active: TypeaheadMatch | undefined) {
    this._active = active;
    this.activeChanged();
  }

This setter however is not being utilized and the logic instead uses setActive when change occurs:

protected setActive(value?: TypeaheadMatch): void {
    this._active = value;
    let preview;
    if (!(this._active == null || this._active.isHeader())) {
      preview = value;
    }
    this.parent?.typeaheadOnPreview.emit(preview);
  }

Firing the event inside setActive appear to fix the issue when testing locally and can be a potential fix.

protected setActive(value?: TypeaheadMatch): void {
    this._active = value;
    let preview;
    if (!(this._active == null || this._active.isHeader())) {
      preview = value;
    }
    this.parent?.typeaheadOnPreview.emit(preview);
    this.activeChanged(); <--- Added
  }