Multisite search doesn't filter by site

Hi everybody,

I've configured apache multisite search module in a test environment.
I've 3 websites

mainsite.local
mainsite.local/subsitea
subsitec.local

all of them share the same code base but each has its own database. In each of them I've configured a multisite index (localhost jetty solr instance - 3.6.2) and I've generate some test contents and indexed.

the metadata info about multisite content seems correct, in fact in each website I see all metadata. In spite of that in the search page (this is true for each of the three websites) the facet multisite block show only the current website:

see screenshot files

What am I doing wrong?

tnks
gmario