I investigated thread leaks in my projects and found the issuer in Firestore.java API
Each usage of BulkWriter release at least one more unterminated thread.

We use driver version

com.google.cloud
spring-cloud-gcp-starter-data-firestore
3.2.1

I know that is legacy version, but still prefer to open the issue since did not found other reports about, as well as did not found release note dedicated to it.