The main problem is the following which is solved partially by EZP-22859. When you have two objects, let's call them Parent and Child.
The Parent object has an objectrelationlist attribute. We add Child objects to that relation list.

We then index these objects in Solr and if we search for Parent filtered by the object relation list containing Child objects it works correctly until the linked Child object is changed.

The search does a text search for the "title" attribute of the Child object which we just changed and hence it will not match anymore. The problem here is that the objectrelationlist datatype is still referencing the old version of the linked object and the indexing process is getting the old metadata.

This was actually pointed out on ticket EZP-22859 as one of the last comments (https://jira.ez.no/browse/EZP-22859?focusedCommentId=101881&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-101881).

A possible solution was to do the following:
To patch the kernel/classes/datatypes/ezobjectrelationlist/ezobjectrelationlisttype.php file at line 1568 with the suggestion from fassino's comment. (See attached patch file)

We also created a new workflow event to run for the above mentioned "Sector" object to get all reverse related objects and reindex them through the ezpending_actions database table and the indexcontent cronjob. Otherwise a full reindex run through eZ Find's updatesearchindexsolr.php script is required to index the changes correctly.

*Steps to reproduce:*

1. Create a "Parent" class with the following settings:
Name: Parent
Class identifier: parent
Object name pattern: <title>
Attributes:
a. Name: Title
Identifier: title
Type: Text line
b. Name: Children
Identifier: children
Type: Object Relations

2. Create a Child class with the following settings:
Name: Child
Class identifier: child
Object name pattern: <title>
Attributes:
a. Name: Title
Identifier: title
Type: Text line

3. Create an object of Child. Set title to "Test child"
4. Create an object of Parent and add Child in the object relation list
5. Index the site in Solr
6. On the Solr admin interface do the following query

http://[url to solr admin]select?q=*%3A*&fq=attr_+children_t%3A%3D"Test+child"&fq=meta_class_identifier_ms%3A%3D"parent"&fl=attr_title_t&wt=json&indent=true&qt=ezpublish

This should return one result, the Parent object created on step 4.

7. Change the title of Child to "Something something"
8. Reindex the site
9. Execute the following in the Solr admin

select?q=*%3A*&fq=attr_+children_t%3A%3D"Something+something"&fq=meta_class_identifier_ms%3A%3D"parent"&fl=attr_title_t&wt=json&indent=true&qt=ezpublish

No results will be returned

10. Apply attached patch to legacy kernel
11. Reindex site
12. Execute query from step 9.
There should be 1 result