I am not sure if this is a bug or issue whether rather feedback.

I have created page, database...

and category, where I filled up Name of category "This is title"

 

On Meta Tags & Title card I see option Category title

 

 

so I filled up this field too and result is:

 

<title>This is title - This is title tag - My site</title>

 

But I would expect in title only This is title tag - My site.

 

Because when I am creating page and fill this:

 

 

and then this:

 

 

result is:

 

<title>My Main page title - My site</title>

 

So Page Title on Meta Tags & Title card replaced page title filled up on Details card. This is how it should works in my opinion.

 

But in case of categories Category title on Meta Tags & Title card was added to original Name filled on Configuration card. If I used the same name in both fields, I would have duplicate content in the category name.

I'm a little confused by what is being reported here, as what you have looks to be correct. It's showing page title - category title - subject 

Then when I create or edit Category I cannot use the same value in Name and Category title fields if I want to avoid duplicate content in title od browser page.

That would be correct, yes.