Please refer the Tag propagation section from the below mentioned docs

Tags are propagated to AWS EC2 instances differently depending on whether or not a cluster was created from a pool.

https://docs.databricks.com/administration-guide/account-settings/usage-detail-tags-aws.html

If a cluster is created from a pool, its EC2 instances inherit only the custom and default pool tags, not the cluster tags. 

Therefore if you want to create clusters from a pool, make sure to assign all of the custom cluster tags you need to the pool.