In previous versions if the child window's position was not specified, the child window was centered according to the dimensions of the work area (SPI_GETWORKAREA), but since child windows are positioned relative to the parent GUI, this always resulted in the child GUI being invisible. Now the child window is centered according to the dimensions of the parent GUI.
The dimensions used for centering were also used for restricting the first Gui Show (if size was unspecified). That being the case, the child window is currently restricted to 0x0 if the parent GUI has not yet been sized. You can work around this by calling
Gui, Show, Hide w222 h222 (or without Hide) before showing the child GUI.