I just did it on an iphone 6S Plus and an iPhone XS both with ios13+, and while in landscape, if I press lock rotation, it changes to portrait and then locks it.

I’ve experienced this since iPhone 3G or iPhone 4, so I assume this is how they have designed it.

https://discussions.apple.com/thread/8237975

This website shows a workaround for it:

https://www.igeeksblog.com/how-to-lock-rotation-in-iphone-la...