GRUB "hangs" on launch when powering on Surface Laptop 3 with Ubuntu, Windows 10

G'day all.

I've installed Ubuntu 19.10 alongside the pre-installed Windows 10 on a Microsoft Surface Laptop 3 (i.e. it is running on an NVMe SSD, with UEFI only). I've set the GRUB (2.04) bootloader to be the default boot option, from which I can boot both Ubuntu and Windows. However, whenever I power on the laptop, after the Microsoft splash disappears and the GRUB interface appears, there is always a 15 second period within which all keyboard input doesn't register. After the 15 seconds, a keyboard symbol appears in the bottom right, at which point GRUB functions perfectly normally.

I have already ruled out a number of possibilities as to what might be causing this. It isn't the setting for GRUB_TIMEOUT in the GRUB config, as the timer that it controls is also frozen and doesn't start to count down until the keyboard symbol appears. The issue occurs whether or not Secure Boot is enabled in the UEFI; it does NOT, however, occur when I boot directly to GRUB from the UEFI - only when pressing the power button with GRUB the default boot option. It also seems to occur regardless of which version of Ubuntu and GRUB are installed; I had the exact same issue with Ubuntu 18.04 and GRUB 2.02.

Would anyone happen to know what is causing this, and what could be done to avoid it? I have a suspicion that this might not be Ubuntu specific, so if it were not, I would appreciate if someone could point me in the right direction regarding where to ask next.

Let me know if you need any more info, or a video showing what is happening. Thanks in advance!