Disclaimer: I haven't used Pulumi to create a GKE Node pool, but if it helps I could help a bit with the troubleshooting steps 😄
• Is there any default being set by the pulumi resources you're using? (what I encountered in GKE terraform modules is that a default node_pool is being created, not sure if this applies in your scenario as well)
• When using preview, are both the name and the name_prefix being displayed? or the error happens during the planning?
• Have you checked the structure to ensure that you're properly overriding the value?