[vim/vim] Bug in code or unclear documentation on 'laststatus' (Issue #20875)

ubaldot created an issue (vim/vim#20875)

### Steps to reproduce

```vim
vim --clean
set laststatus=2
:vsplit
:split
<c-w>k
set laststatus=0
```

I am not sure if this is a bug or am I misinterpreting the documentation. 
Note that on the top-right window `:echo winnr()` returns 1. 

### Expected behaviour

The statusline of the top-right window disappear. 

### Version of Vim

MS-Windows 64-bit GUI/console version with OLE support Included patches: 1-45

### Environment

Windows 11, Windows Terminal and gvim

### Logs and stack traces

```shell

```

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