I am experiencing a visual issue with the GRUB bootloader.

When I start my computer, the GRUB boot menu is displayed fullscreen as expected. However, after I select an operating system, a black rectangle appears for about 2–4 seconds before the system continues booting normally.

I noticed that when I set GRUB_BACKGROUND, the black rectangle is replaced by the selected background image.

My current GRUB configuration is:

GRUB_GFXMODE=1920x1080
GRUB_GFXPAYLOAD_LINUX=keep
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

The boot menu itself is displayed correctly in fullscreen, only the transition phase after selecting the OS shows this black rectangle / background image.

Questions:

What causes this rectangle to appear during the boot transition? How can I eliminate this visual bug so the transition is seamless?

Any help or explanation would be appreciated.