My use case was slightly different so your update didn’t work for me, but I found a slightly different solution that does the trick:
@ViewChild(MatAutocompleteTrigger) autocomplete: MatAutocompleteTrigger;
Then you can use this to close the dropdown options:
this.autocomplete.closePanel();
Make sure to also import ViewChild:
import { ViewChild } from '@angular/core';
Works like a charm.