Same issue with a Lenovo T450s (integrated Intel graphics) with a fresh 20.04 install next to Windows.
At first, if I entered the bios config and then exited, I could get the grub menu as expected with a high resolution.
I tried Alex Barris' suggestion and changed to Legacy/UEFI from UEFI only and now I get a low resolution splash screen and see the grub menu. This is true even if I change my boot config back to UEFI only. So it's working, but now stuck in a low resolution.
Same issue with a Lenovo T450s (integrated Intel graphics) with a fresh 20.04 install next to Windows.
At first, if I entered the bios config and then exited, I could get the grub menu as expected with a high resolution.
I tried Alex Barris' suggestion and changed to Legacy/UEFI from UEFI only and now I get a low resolution splash screen and see the grub menu. This is true even if I change my boot config back to UEFI only. So it's working, but now stuck in a low resolution.