Third-party app not in landscape mode with Magic Keyboard
I have the same problem with the Lastminute.com app. Its a great travel app, but refuses to go into Landscape mode so I can’t use it with my magic keyboard.
iPad Pro, iPadOS 26
If screen rotation proves to be problem with some Apps - and not others - you need to be aware that not all Apps are coded to operate in both Portrait and Landscape screen modes. Apps that operate in full-screen with a fixed screen orientation will always* appear in the orientation determined by the App Developer.
(*) For compatible Apps that are intended for iPhone, iPadOS15 will automatically rotate the App to display in Landscape screen orientation. There is no manual configuration or setting to enable this feature.
However, for Apps designed for iPad that are expressly coded to force Portrait screen orientation, iPadOS will honour the intended Portrait mode - and will not override the App coding.
Tags appended to your post suggest that your iPad has iPadOS 26 installed. iPadOS 26 supports windowing - and can often display a Portrait-orientation App within its own portrait window while the iPad itself is in landscape orientation.