This has been reported to Alacritty in alacritty/alacritty#2561.

On X11, it seems like setting the startup mode after the window is show()n on an originally invisible window has some inconsistent behavior. I've tested this myself on i3.

When the window is created and set to fullscreen on window.get_current_monitor() right at startup, the window is fullscreened on the incorrect monitor.

Additionally, if the position is set right after show() too, the window is fullscreened on the incorrect monitor and when leaving fullscreen mode the position is not what it has been set to.

If set_fullscreen is manually called on a keyboard event at a later point in time, both of these work without any issue.

See #978 (comment).