Same terms, different blogs : no relationship stored

Ok, I've come to Post indexer after looking for many different solutions.
The trick here is :
- 2 websites (there will be a third) with WooCommerce 'product' and 'product_variation'
- 1 main website that needs to show (and sort, display, etc.) the product lists for all sites. No need for product page (that would create duplicate content) but only for the lists.
- 2 custom taxonomies ('theme' and 'country'), the same for every child site. These are used on the main site for navigation and listing purposes.
- post_indexer settings : post_type = { post, product, product_variation }
- sites settings : do not index main site (which has no content at this time)

Example :
product (post_id 222) on blog_id 2 has 'theme' = 'trekking' , 'country' = 'Nicaragua'
product (post_id 333) on blog_id 3 has 'theme' = 'yoga', 'country' = 'Nicaragua'

Behaviour :
Both products are properly indexed (including product_variation children) but not their relationships.
In the network tables I get :
blog id 2, object_id 222, taxonomy 'theme' , name 'trekking'
blog id 2, object_id 222, taxonomy 'country', name 'Nicaragua'
blog id 3, object_id 3, taxonomy 'theme' , name 'yoga'
And that's it.

Expected behaviour : I should get another record :
(blog id 3, object_id 333, taxonomy 'country' , name 'Nicaragua')

For reference, the query is :

SELECT tr.*, t.name, tt.taxonomy FROM <prefix>_network_term_relationships tr
JOIN <prefix>_network_term_taxonomy tt ON tt.term_taxonomy_id = tr.term_taxonomy_id
JOIN <prefix>_network_terms t ON t.term_id = tt.term_id
WHERE tt.taxonomy = 'country'
ORDER BY tr.blog_id, t.name;

I double checked the relationships :
On blog_id 2, there are 12 products, each having a relationship to one 'country'
On blog_id 3, there are 12 products also, 12 relationships.
Between blog_id 2 and 3, 4 'country' are the same.
In network_tables I got 24 'product' objects (that's good). But only 20 relationships between objects and 'country' terms. No relationships were stored for the 4 products of blog_id 3 having a 'country' existing on blog_id 2.

In other words, when two objects from two different blogs are linked (on their respective blogs) to the same (homonym) term, only objects from the first blog got their relationships stored (in the network_tables). Whether it's a problem with storing the same term from the second blog, or just storing the relation ship between a blog_id 2 term and a blog_id 3 object , I don't know.

I know this particular need is both not in the original scope of this plugin, but quite useful for many shop managers.
Is there a way to improve this behaviour ? (or is just something I've overlooked ?)