I've understood now, done a test, you are right. Thanks for helping, so we found the bug, I hope it will be solved. I am just a user, not a programmer, so I can't help further.

I confirm, I've tested again, the problem is in Cyrillic characters in search request, not in Mediawiki localization. Resolves if

$wgAdvancedSearchHighlighting = "true"/"false";

is set.

On my dev wiki, with the default value of $wgAdvancedSearchHighlighting I only get titles. This is expected behaviour.

As an aside, I don't know why you're setting it equal to "true" or "false"; true or false should be used as they are bools.

It is defined in DefaultSettings (like every other MW Core config) https://github.com/wikimedia/mediawiki/blob/REL1_36/includes/DefaultSettings.php#L7046-L7050

The code that uses it does

This has nothing to do with Wikimedia sites (!) in Russian language

I tried to install English wiki on the same config and everything works OK. So, this problem is Russian localization-related.

I think I'm going to reopen this, so maybe someone can repeat the bug or tell me about some checkings I can do. I really don't think that the problem in that tool, there's something wrong with MediaWiki.

Whatever "SoftAculus" is, we have nothing to do with that. Feel free to ask in support forums instead.

I don't really think that the problem is in it, Softaculous Auto Installer is an auto installer soft that create identical to manually installed wiki, many hoster providers use it. This soft doesn't modify wiki files except for DB and folder variables.

The strange thing is that

$wgAdvancedSearchHighlighting = "false";
give the same result, while the absence of this variable leads to the absence of text exceptions in search results at all.

It was not so evident, but the solution was very simple: you have to set

The latest version of the extension still doesn't work on MediaWiki 1.36 with the same problem.

In T286749#7216212, @Reedy wrote:

What version of MediaWiki? What version of TemplateStyles?

This was fixed in rETST24cb62e04457: Update use of css-sanitizer classes for PHP 8.0 compat, should be fine in 1.35, 1.36 and master...

In T286710#7214802, @Aklapper wrote:

MediaWiki 1.34

Hi @Arskrigicioniec, you are using an old outdated version which is too old and not supported anymore. Developers are no longer working on that version, so unfortunately there will not be any bug fixes by developers for the version that you use.
If you don't plan to upgrade to a secure and supported version, then please feel free to bring this topic on https://www.mediawiki.org/wiki/Project:Support_desk

If the problem still occurs with a newer and supported version of MediaWiki, please set the status of this Phabricator task back to "Open" via the Add Action... → Change Status dropdown, provide exact version information, and follow https://www.mediawiki.org/wiki/How_to_report_a_bug . Thanks!

I have the same problem with my MySQL shared hosting on clean 1.34 MediaWiki. I use Russian local. The search results give me only headers, despite of search by key words actually works.