The Microsoft RDP android app does not yet support portrait mode...

The good news is that this feature is listed: https://remotedesktop.uservoice.com/forums/272085-remote-desktop-for-android/suggestions/6851819-portrait-mode

So lets vote for it and then hope the feature will be available soon as this feature has the most votes already...

The Microsoft RDP android app didn't support portrait mode... but with your input on https://remotedesktop.uservoice.com/forums/272085-remote-desktop-for-android/suggestions/6851819-portrait-mode it finally got implemented:

Our latest update to the app supports connecting in portrait mode. The remote session adapts to your device orientation dynamically. You will get the best experience using Windows 8.1 and newer.