The accept() method of the JPAPlaceholderResolverStrategy is accepting any object that is an @ Entity despite its EntityManager does not handle these entities.
The current approach only uses reflection to guess if an object is an entity.

It is possible to validate which entities handles an EntityManager asking for the metamodel:
if( em != null ){
Set<EntityType<?>> entities = em.getMetamodel().getEntities();
for( EntityType<?> entity : entities ){
if( varClass.getCannonicalName().equals( entity.getJavaType().getCanonicalName() )

}
}

This bug, linked with DROOLS-1629 is limiting the posibility that a project uses two different datasources in the same runtime environment.