[3.0]: Custom search index weirdness

[Oldiesmann]

Basic Information

While playing around with the search the other day trying to reproduce #7870 I discovered some weird issues with the custom search index - sometimes it doesn't return any results when it should and other times it doesn't return the expected results.

I have the following on my board:
A post with the word "testing"
A post with the phrase "trying to reproduce the error"
A post that quotes the "error" post (same topic)

Search tests:
Searching for "testing" - no results
Searching for "trying" - no results
Searching for "reproduce" or "error" - single result in the quote (but not the post that is being quoted)

Steps to reproduce

1. Create some topics/posts
2. Create a custom search index
3. Attempt to search for various words within the posts/topics created

Expected result

Expect to find all posts/topics containing the word searched for

Actual result

Inconsistent/unexpected results (see above)

Version/Git revision

a540d9e

Database Engine

MySQL

Database Version

10.11.7-MariaDB-1:10.11.7+maria~ubu2204

PHP Version

8.3.6

Logs

No response

Additional Information

The MariaDB version doesn't seem to have any play in this. I initially ran into these issues while on MariaDB 10.6 but upgraded to MariaDB 10.11 last night (not because of this problem but because Plesk now has a feature to upgrade to the latest LTS version).
The size of the custom search index doesn't seem to make a difference here either, though this forum only has 6 posts in 3 topics so I doubt there's much if any difference between the actual index size regardless of which option you choose when creating the index.
This occurs both with the basic search and the advanced search.