Don't know if it is an issue, it must cost a lot of dev for a feature which may never be used.
So we can close this issue if it is not interesting.

• Solr (in AmetysSimpleFacets) calls FunctionValues#strVal, so it is not currently possible to have a final field which is multivalued (in fact it is possible, but only the first value is taken and it is considered as it is a singlevalued field)
• As a consequence, we do not allow to have joined criteria (see org.ametys.cms.search.model.SystemProperty.isFacetable(SystemProperty, boolean) for instance) on mutlivalued system properties or metadata.
• A solution could be, in Solr, to call FunctionValues#objectVal (in AmetysSimpleFacets). Then, JoinValues#objectVal should be modified and be smart, returning sometimes a String, and sometimes a Collection.
• What's more, currently, if in the join-path, a Solr field represents an Ametys multivalued metadata (of type CONTENT of course), then we only follow the first joined content. As we allow this kind of path in Ametys side, this is clearly a limitation of the feature.