ピンが自動的に削除されましたか?

I found weird stuff.

I pinned the topic on the top and I read it then the pin automatically unlocked, the topic not on the top anymore. (logged in as admin)

But the topic still pinned on top if I visit as non registered user and always stay on top no matter how many times I read the topic.

why is it like that?

ピン留めされたトピックは、既読になると自動的にピン留めが外れます。匿名ユーザーには既読トラッキング機能がありません。データベースの容量が足りないためです。そのため、匿名ユーザーにとってピン留めされたトピックは「既読」としてマークされることはありません。

If you don’t want that behavior you can go to your
https://meta.discourse.org/my/preferences/interface
and uncheck the “Other - Automatically unpin topics when I reach the bottom.” option

that’s the option I was looking for but it only applies to my account?
how can I apply this setting to all the members?

There’s an automatically unpin topics option in your site settings

can you guide me in which section it exists?
I can’t find

/admin/site_settings/category/all_results?filter=unpin

But that only applies to new members who join, to update existing members you have to use a rake task and update their preferences.