Actually there's no bug

It works fine.. starts always in landscape mode, but rotates if phone is already in portrait.

We'll check phone rotation on start up to speed it up.

The color chooser doesn't work in portrait and forces the phone in landscape, but on closing it, Xournal returns in portrait mode.
Few buttons text looks truncated in Portrait, will shorten them up.

A part from them , the only things missing in Portrait is Fullscreen button. You can use CTRL+F but is not convenient.

Luckily you can use Xournal in portrait with keyboard open.. it looks quite useful indeed.. might bind several action to keys and they'd be there under your thumb.

Aniello