I have isolated this issue with the following component, you can reproduce by using firefox, here is a video showing the issue: https://drive.google.com/file/d/1f5iI51jk_TEd1ZpszPa-u3OL92IO7Gb5/view?usp=drivesdk

component.ts

import { Component, OnInit  } from '@angular/core';
import { FormControl, FormGroup } from '@angular/forms';
import { Observable } from 'rxjs';
import { map, startWith } from 'rxjs/operators';

@Component({
  selector: 'app-upload-main',
  templateUrl: './upload-main.component.html',
  styleUrls: ['./upload-main.component.scss']
})

export class UploadMainComponent implements OnInit {
  suggestions: any;
  constructor() { }

  results: Observable<string[]>;
  form: FormGroup;

  ngOnInit(): void {
    this.form = new FormGroup({
      textAreaControlName: new FormControl('')
    });

if (!this.suggestions) {
  this.suggestions = [''];
    }

this.results = this.form.controls.textAreaControlName.valueChanges.pipe(startWith('this is a suggestion'), map((value: string) => this.filter(value)));
  }  

  filter(value: string): string[] {
      let filterValue: string;
      if (!value) {
        filterValue = '';        
      } else {
    filterValue = value.toLowerCase();
  }

  if (this.form.controls.textAreaControlName.value) {
    return this.suggestions.filter((item: string) => item.toLowerCase().includes(filterValue));
  } else {
    return this.suggestions;
  }
  } 
}

component.html

 <div class="md-form mt-3">
   <textarea
  mdbInput 
  mdbValidate 
  [mdbAutoCompleter]="auto"
  type="text"
  class="completer-input mdb-autocomplete md-textarea form-control">
  </textarea>  
  <mdb-auto-completer [disabled]="disabled" #auto="mdbAutoCompleter" textNoResults="" [clearButton]="true" [appendToBody]="true"> 
    <mdb-option *ngFor="let option of results | async" [value]="option">
      <div class="d-flex justify-content-left align-items-center w-100">
        <mdb-icon *ngIf="multipleSelected" far icon="images" class="align-self-center mx-1">        </mdb-icon>
        <span  class="align-self-center mx-1">{{ option }}</span>
      </div>                  
</mdb-option>
  </mdb-auto-completer>
</div>

Did you ever find a fix for this? I am having the exact same problem?

Code below

  <div class="md-form mt-3" *ngIf="inputType === inputTypeEnum.TEXTAREA">
<textarea
  mdbInput 
  mdbValidate 
  type="text"
  class="completer-input mdb-autocomplete md-textarea form-control"
  ngClass="{{textAreaClass}}"
  formControlName="textAreaControlName"
  [mdbAutoCompleter]="auto"
  [placeholder]="textAreaPlaceholder"
  (focus)="onTextAreaFocus()"
  (focusout)="onTextAreaFocusOut()"
  (change)="onAnythingChanged(false)"
  (clearBtnClicked)="onAnythingChanged(true)">
</textarea>
<mdb-error *ngIf="textAreaControl.invalid">
  <span *ngFor="let e of textAreaControl.errors | keyvalue">
    {{e.value}}
  </span>
</mdb-error>    
<mdb-auto-completer [disabled]="disabled" #auto="mdbAutoCompleter" textNoResults="" [clearButton]="true" [appendToBody]="true" (selected)="onAnythingChanged(false)"> 
  <mdb-option *ngFor="let option of results | async" [value]="option">
    <div class="d-flex justify-content-left align-items-center w-100">
      <mdb-icon *ngIf="multipleSelected" far icon="images" class="align-self-center mx-1"></mdb-icon>
      <span  class="align-self-center mx-1">{{ option }}</span>
    </div>                  
  </mdb-option>
</mdb-auto-completer>