As discussed in LibreOffice Design's Telegram group, I would like to report Muhammet Kara's finding to let him fix the bug he have found. :)

So the "Orientation" command in Writer is a toolbox with two options : Portrait and Landscape. Strangely, these options are not persistent with their respective naming. They are basically do the same thing, toggling back and fort against page orientation.

So let me explain the step to reproduce

1. Open a new Writer document
2. Add "Orientation" command to any toolbar (Tools > Customize..., select Toolbar, search for "Orientation", click right arrow to add to the Target toolbar or right click at any active toolbar, click Customize Toolbar..., search for "Orientation", click right arrow to add to the Target toolbar)
3. By default, Writer page is portrait, then try to click "Portrait" option under Orientation toolbox, and boom! The page become landscape.
4. Go to Page sidebar, in the Orientation drop down option under Format section, the text showing Portrait (following our choice in step 3: Portrait), actually the page now is landscape.
5. Now click "Landscape" option under Orientation toolbox, the page actually become portrait and the text in the Page sidebar become Landscape (following our choice :"Lanscape)

One can assume user rarely adding "Orientation" command to the toolbar and the Orientation in the Page sidebar do the job finely. But as Muhammet said, the sidebar is a mapped implementation of the command so we should fix this strange Orientation command behavior before the situation get worse.