Strange black background behaviour in ionic/angular application

I have just updated my app to ionic 5 and angular 9. I was hoping that this problem will disappear with the new version but it still persists. The issue is that in my app all are pages transitioning with a black screen before the real page comes in. So the user would see for like some millisecond a black screen, what really does not look nicely. Now with the new version things got even worse. In some pages I also experience the black boxes in the ion-refresher area or when opening the keyboard. As well when opening the new modals(in some its working properly in some not) But with the modals the black background even stays behind the modal. I really have no idea why this is happening. I also don't know if code helps here because its actually the same in both modals or also for refreshing content. I have made two pictures of an correct working modal and one with the black background.

I appreciate any help and if you need further code, let me know!

further code:

working modal: html:

<ion-header>
  <ion-toolbar>
    <ion-buttons slot="end">
      <ion-button color="medium" (click)="dismiss()">
        <ion-icon slot="icon-only" name="close"></ion-icon>
      </ion-button>
    </ion-buttons>
    <ion-title color="medium">
      Privacy Policy
    </ion-title>
  </ion-toolbar>
</ion-header>

<ion-content class="legal-content">
<p> ... </p>
</ion-content>

ts:

  async showPrivacyModal() {
    const modal = await this.modalController.create({
      component: PrivacyPolicyPage,
      presentingElement: this.routerOutlet.nativeEl,
      swipeToClose: true
    });
    return await modal.present();
  }

buggy modal: html:

<ion-header>
  <ion-toolbar>
        <ion-buttons slot="end">
            <ion-button color="medium" (click)="dismiss()">
              <ion-icon slot="icon-only" name="close"></ion-icon>
            </ion-button>
        </ion-buttons>
    <ion-title color="medium">
       Black BG
    </ion-title>
  </ion-toolbar>
</ion-header>

<ion-content>

</ion-content>

ts:

  async openFilter() {
    const modal = await this.modalController.create({
      component: FilterPage,
      presentingElement: this.routerOutlet.nativeEl,
      swipeToClose: true
    });
    return await modal.present();
  }

Here the pictures: