See https://ci.hibernate.org/job/hibernate-search/job/master/462/testReport/junit/org.hibernate.search.documentation.search.projection/ProjectionDslIT(default-default)/lucene_elasticsearch_/

It occurs randomly. Probably caused by concurrent access to a Gson object.
I suspect it's somehow related to changes in HSEARCH-3084 Resolved .
Maybe we've been using Gson wrong all along? Maybe it's not supposed to be used concurrently?