Practice exam answer claims “Deploy existing reserved instances into a Placement Group” does not incur any costs

This is probably true in most cases, but what if I deploy a zonal reserved instance (for e.g. eu-west-1b) into a cluster Placement Group that already has a member on another AZ (e.g. eu-west-1a)?

Availability Zone of the instance changes, and I will pay both on-demand cost + the [non-utilized] reserved instance cost for the moved instance.