iPad landscape mode is not available anymore

You are right

For the most up-to-date and accurate information about iPad features and troubleshooting, it's advisable to refer to Apple's official documentation or contact technical assistance if you're still having issues. My Singing Monsters My Singing Monsters

With the new update, the app now functions normally and supports both portrait and landscape modes on my iPad Pro 11" 1st generation running iPadOS 15.1.
fnf

If you're experiencing an issue with the latest update of an app where it only works in portrait mode on your iPad Pro 11" (1st generation) running iPadOS 15.1, there are a few steps you can try to resolve the issue:
Check for App Updates: Make sure you have the latest version of the app installed. Sometimes developers release updates specifically to address compatibility issues with new operating system versions. Open the App Store and go to the "Updates" tab to check for any available updates for the app in question. Dave The Diver