Thanks JF, I assumed the spare would automatically replace the removed drive, rather than replacing the spare. This is how I understood the manual.
9.2.5 Adding Spare Devices ZFS provides the ability to have “hot” spares. These are drives that are connected to a pool, but not in use. If the pool experiences the failure of a data drive, the system uses the hot spare as a temporary replacement. If the failed drive is replaced with a new drive, the hot spare drive is no longer needed and reverts to being a hot spare. If the failed drive is instead removed from the pool, the spare is promoted to a full member of the pool