Nodepool has an hardcoded 1 minute delay before deleting an instance. Can be found in the source with the global DELETE_DELAY.
The reason is that OpenStack has an async publisher that grabs the console log and is executed after the build is completed. Deleting the instance as soon as the job is completed would cause the console to no more be available apparently.
Nodepool needs a configurable setting for that delay. To be proposed upstream.