Unfortunately the decision to minimize on a loss of focus is up to the game itself, so that part is not an Ubuntu bug.
What is an Ubuntu bug is that desktop-icons-ng still steals and then quickly restores focus when an app goes fullscreen. This is harmless to most fullscreen apps but obviously not to some games.
A proper fix might end up coming from mutter rather that desktop-icons-ng. I'm not sure yet.
Unfortunately the decision to minimize on a loss of focus is up to the game itself, so that part is not an Ubuntu bug.
What is an Ubuntu bug is that desktop-icons-ng still steals and then quickly restores focus when an app goes fullscreen. This is harmless to most fullscreen apps but obviously not to some games.
A proper fix might end up coming from mutter rather that desktop-icons-ng. I'm not sure yet.