Hello Julien Benichou,

I'm not very familiar with this part of the codebase (yet), but this looks like either a bug or a missing feature. I'd be tempted to call this a bug, since we have code in buildDocumentFieldsForProperties that hints at some support for multi-valued facets:

						for ( FacetMetadata facetMetadata : fieldMetadata.getFacetMetadata() ) {
							if ( multiValued ) {
								faceting.setMultiValued( facetMetadata.getFacetName() );
							}
							addFacetDocValues( document, fieldMetadata, facetMetadata, currentFieldValue );
						}

I'm currently having some time off, but I'll have a look when I come back next week.

Cheers,
Yoann