Alien Hominid HD
I also can confirm this has been an issue since release. The mouse pointer is not seen on Steam Deck/SteamOS, but it is seen on other linux distros. I also tried launching the game from Big Picture mode, and in both windowed and full screen settings, but the problem persists. I am able to get the mouse pointer to disappear when in full screen mode by alt-tabbing out and back into the game. In my case, I am running Manjaro with KDE 6.05, but I also experienced the problem under KDE 5.x as well.
Also worth noting is the mouse pointer is initially not on screen. When actually starting the game, the mouse pointer is re-positioned to to dead center of the screen and is held in place. When moving the mouse, the pointer can be seen moving for a frame or two, but then is brought back to dead center of the screen, like an insect right in the middle of the screen that you can't get rid of. (not without alt-tabbing or minimizing as cyo93 suggested)
The Alt-tab work around does not work in borderless windowed screen mode - alt-tab only removes the mouse pointer when the game is in full screen mode. In windowed mode the game is completely unplayable due to bad flickering and graphic slowdown. These behaviors are the same in both X11 and Wayland sessions.
Nothing extracted yet.
Compatibility Report
System Information
I confirm:
Symptoms
The mouse cursor doesn't hide properly in-game unless I minimize the window at least once.
Reproduction
Start the game and play a level. If you haven't minimized the application, the mouse cursor will be stuck at the center and not be hidden until the game is minimized at least once.