This may be related to the fact that the faceting field and another regular field may have the same name, but have different mappings (most notably the facet field is not analyzed), which means that the Elasticsearch mapping will be the one for whichever field we inspect last.
See https://github.com/hibernate/hibernate-search/blob/55d753e/orm/src/test/java/org/hibernate/search/test/query/facet/Car.java#L57..L64 for the conflicting regular/faceting fields: we have a "make" regular field, which is not analyzed, a "facetingNameCollision" regular field which is analyzed, and a facet named "facetingNameCollision" on the "make" regular field.
See https://github.com/hibernate/hibernate-search/commit/34f7eaafadeabbb75597306b20258db7ce198850 for the original commit introducing the @Facet annotation, that states in its commit message that facet fields should not be analyzed: "Adding verification that indexed field configured for faceting is not analyzed"
The documentation states "In order to facet on a given indexed field, the field needs to be configured with the `@Facet` annotation. Also, the field itself cannot be analyzed.", and we we already throw an exception when we find a @Facet annotation on an analyzed field, but we don't detect name collisions between a facet and an unrelated field...
Maybe we should always add a suffix or prefix to faceting fields, like "<field name>_HSearch_facet"? Or we should detect such name collisions and throw exceptions?
StringFacetingTest.testStringFacetingWithNameCollision() has been disabled for Elasticsearch and should be enabled again when this is solved.