ionic 4 + angular: routerLink only works first time

This is already fixed, just run npm i @ionic/angular to update to the fixing version 4.2.0 or superior.

This was also affecting router.navigate(['url', params]) functionality

Solved it by using navigateByUrl instead like this:

open(id: number) {
  this.router.navigateByUrl(this.router.url + '/' + id);
}

and for the card:

<ion-card *ngFor="let item of items;" (click)="open(item.id)">
    ...
</ion-card>

Still don't know why this is happening but this works as a workaround for now.

This is a known bug. You can find the GitHub issue here: https://github.com/ionic-team/ionic/issues/16534.

As @mario above mentions, absolute links will work with routerLink. See comment here: https://github.com/ionic-team/ionic/issues/16534#issuecomment-444610330