Ion-item highlight border is wider than the inset border

I’m using <ion-item lines="inset"> in an ion-grid (Ionic 4), and the bottom border looks like this on a <ion-input>:

Capture1

But when the item has the input focus, the focus highlight is wider than the inset line:

Capture2

How can I make the highlight border the same width as the unfocussed inset border?