A ion-input needs to be clicked twice if it is located in a ion-item besides a ion-input with html below:

<ion-item-group>      
    <ion-item (click)="onClick()">
        <ion-input type="text" placeholder="input something"></ion-input>
     </ion-item>
</ion-item-group>

So that the solution is

<ion-item-group>      
    <ion-item (tap)="onClick()" (press)="onClick()">
        <ion-input type="text" placeholder="input something"></ion-input>
     </ion-item>
</ion-item-group>