[5.9.15]: Search index not updated when related entry title changes

[thupsi]

What happened?

Description

When changing the title of an entry referenced in an entries field, the search index for the entry containing that field is not updated.

Steps to reproduce

1. Have 2 entries, with titles Entry A and Entry B respectively.
2. Setup an entries field and enable the Use this field’s values as search keywords.
3. Add this field to the field layout of Entry A.
4. Go to Entry A and in the field we just created add Entry B.
5. In the entries index, when you search for Entry B, both Entry A and Entry B show up.
6. Update the title of Entry B to Entry C.
7. Now, in the entries index, when you search for Entry C, only Entry C shows up.

Expected behavior

Search indexes should pickup changes to related entries' titles and update accordingly.

Actual behavior

The changed title of the related entry does not get picked up.

Craft CMS version

5.9.15

PHP version

No response

Operating system and version

No response

Database type and version

No response

Image driver and version

No response

Installed plugins and versions

[linear]

CMS-1886

[thupsi]

I should add that when I resave entries with the update-search-index flag, everything works as expected.