It’s very common to have this be an issue with EZ heads.

I usually do a bit of relieving on the EZ’s I sell.

1.5’s often rub on the side of the slot towards the manifold at higher lifts.
1.6’s often rub on the side of the slot towards the rocker shaft when the valve is closed.

Why this hasn’t been taken care of during manufacture is just another mystery of the car hobby.