5.4 kernel with Intel
Screen oscillates between 1920x1080 and 1920x540 with a yellowish cast.
5.8 kernel with Intel or Nvidia
Screen is a steady 1920x540 with a yellowish cast. Lower half of screen is just vertical bars of varying colors.
I found a person posting on the HP community site with the same issue
HP Envy 15t (2020) display issues on Linux. The included screenshot looks just like what you posted for the 5.8 kernel, except there is no yellowish cast.
It appears your laptop has OLED technology for the display,
Should You Buy an OLED Laptop? Here’s What You Need to Know. The Dell laptop mentioned in that article had a lot of unhappy customers based on what I found on the Dell Community site, although they were experiencing banding issues and not the squished screen.
Did you notice any screen issues while in BIOS/UEFI? If not, then it may be something in how Linux is handling the display. The fact Windows was not having the issue seems to indicate the Windows driver is set up to better handle how the hardware is designed.
Two options that I can think of at this time to try: different kernels and an external monitor
External Monitor
Do you have an external monitor available that you could test? You would most likely need to have the computer in On-demand or Nvidia mode. Most computers with an Nvidia card are set up to run external displays only with Nvidia.
If the external screen displays properly, then it is likely an issue with how Linux is handling the laptop screen. If both do the same thing, then it is likely something in the video hardware.
Different kernels
Since there did seem to be a difference on what you saw between the two kernels you tried, here are some more options to test.
Ubuntu currently has two oem kernels available, 5.6-oem and 5.10-oem. Here is some explanatory information on
OEM Kernels.
Some people with very new Intel hardware have needed to use the 5.10-oem because it has a patch specifically for the i915 Intel graphics driver which matches their very new cpu. I do not know if that might make a difference on your computer since it seems the display looks the same whether you use Nvidia or Intel. Testing them would not take that long and you know how to undo a kernel change if they do not help.
Mint is set up to automatically boot to the highest kernel available, so trying the 5.6-oem and then the 5.10-oem would mean one less boot into grub if you want to undo the changes.
These two kernels are not in Update Manager, but they are in the Synaptic Package Manager app or you could install them from the command line. Notice there is only one letter difference (at the end) between the names for the kernels/commands. You will need to reboot after installing the kernel for it to take effect.
You can install the latest 5.6-oem with
sudo apt install linux-oem-20.04
.
You can install the latest 5.10-oem with
sudo apt install linux-oem-20.04b
.
They are in Synaptic Package Manager as
linux-image-oem-20.04
and
linux-image-oem-20.04b
if you prefer to install through Synaptic to avoid possible typing errors. After you have installed them, they will show up in Update Manager should you wish to delete them.
The last option is to try one of the LM19 versions. Normally I would not recommend going to an older version since your laptop is not that old, but the screen was slightly better with the 5.4 kernel so maybe going older might be better?
The ISO for LM19.2 has the 4.15 kernel. People running LM19 are using either the 4.15 kernel or the 5.4 kernel. The latter is the kernel with LM20, so if kernel is the issue then you have already tried that. (The LM19.3 ISO has the 5.0 kernel which is no longer supported so you would need to switch to the 5.4 or the 4.15 kernel.)
Those are the options I can think of at this time. If you decide to test them, let us know the results of each.