Confirmed as MessageCache related issue:

mwscript eval.php trwiki
> echo wfMessage( 'ipbreason-dropdown' )->inContentLanguage()->plain();
*Genel engelleme nedenleri
** Yanlış bilgi eklemek
** Sayfalardan içeriği çıkarmak
** Dış sitelere spam bağlantı vermek
** Sayfalara mantıksız/anlaşılmaz sözler eklemek
** Tehditvari davranış/Taciz
** Birden fazla hesabı kötüye kullanmak
** Kabul edilemez kullanıcı adı

The page has not been edited recently: https://tr.wikipedia.org/w/index.php?title=MediaWiki:Ipbreason-dropdown&action=history

Potentially an issue with Turkish ucfirst. For example, manually constructing url such as https://tr.wikipedia.org/w/index.php?title=MediaWiki:ipbreason-dropdown&action=history goes to different page.

Is there any chance we can disable caching for a particular page? This issue is blocking trwiki now.

Ah, right. Core interfaces message are keyed in English, not content language. So we'd need to use plain ucfirst() instead.

Except for custom messages used via gadgets and int, it might be different. Since MediaWiki does by default redirect page titles to content language ucfirst, not English. E.g. https://tr.wikipedia.org/w/index.php?title=MediaWiki:ipbreason-dropdown&action=history (English lowercase "i") renders for https://tr.wikipedia.org/w/index.php?title=MediaWiki:%C4%B0pbreason-dropdown&action=history (Turkish uppercase I).

A way to fix this issue was mentioned in a comment: "Use English ucfirst() in NS_MEDIAWIKI instead of content language ucfirst". The other option is just document this (where?) as expected behavior.

Can reproduce for azwiki

In T118684#10512912, @jhsoby wrote:

This would have been solved by this change. Closing this as duplicate even though this task is older, since the fix was attached to the other task.

I just rechecked and it is not fixed

Change #1116533 had a related patch set uploaded (by Jon Harald Søby; author: Jon Harald Søby):

[mediawiki/core@master] Don't use language-specific ucfirst for interface messages

https://gerrit.wikimedia.org/r/1116533