Well it does center it, just there is some padding that exists to save room for any icons you might have. You have to look carefully to spot it.
One option would be to set the mode value on the toolbar like:
<ion-header>
<ion-toolbar mode="md">
<ion-title >
Ionic Blank
</ion-title>
</ion-toolbar>
</ion-header>
or change the padding values on the <ion-title>
in the scss file:
ion-title{
-webkit-padding-start: 0;
padding-inline-start: 0;
}
but if you have any icons that might get messy.