Pymouth is the Ubuntu application that displays the graphical splash screen when booting and shutting down the system^[1]. It is long known that it has problems with proprietary Nvidia drivers. While on some computers it starts in low resolution mode, on others it works in text mode. It is claimed that this happens because Nvidia drivers load at a later point and are not available for GRUB and Plymouth^[2]. Usually, Plymouth uses KMS (Kernel Mode Setting) to display graphics, but with proprietary drivers you must configure it to use framebuffer instead^[3].

Source: Fix boot splash screen (Plymouth) on Ubuntu · One Transistor