Someone with an R12 (same chipset boaed) had indicated this problem comes up with the most recent BIOS update for rebar.  But I don't see that available on the R11 page.  What version are you on?

Clarify, when our OEM RTX 3080 video card was installed, could you see the post screens and F2 BIOS?

Yes, I am aware of this issue. I was trying to look for the excellent post from @speedstep  that explains this issue in great detail, but I could not locate it.

I could locate this Intel post that provides some background information: GOP UEFI 

It has to do with the UEFI pre boot environment (BIOS), and the GOP driver needed to enable video output in that environment. If the video card does not have the correct GOP driver, it will not display any video output in the UEFI BIOS environment.

The only way around that is to enable CSM, and that requires Secure Boot to be disabled. The R10 has that option, but I do not believe the R11/R12 have it.

I remember the post from Speedstep explaining the reason for this, and if I was not mistaken it has to do with the version implementation of Secure Boot. I think version 3 or higher does not allow legacy mode anymore.

Secureboot issue is a boot block (black screen) hard stop no boot to windows.

This is asking about a BIOS no video but boot is OK. This is a different issue. 

It’s not only aftermarket video cards. My 3080 only shows the bios using hdmi and not DisplayPort. The only aftermarket stuff I have is fans and h60 cooler

This is what needs to be turned off to enable legacy boot mode and legacy video card support. Secure boot is the issue with the no video when accessing the bios.

As far as I know the R11/R12 does not have this as an option in the bios menu. I don't have an R11/R12 to test this out, maybe someone who has one can verify this.

The picture below is from my R10 bios menu.

You need to disable secure boot, but that will be difficult without being able to enter the bios and it cannot be done under windows and I don't think the R11 bios has the option to turn secure boot off. You will likely also need to enable CSM, but I believe nether the R11 or R12 have that option. The R10 does have that option in bios.

R11/R12 bios menu: R11/R12 bios menu 

R10 bios menu: R10 bios menu 

The R11/R12 is missing this option in the BIOS: 

It's a common issue with newer systems, because by default they come with secure boot and no way to disable it and enable legacy BIOS mode.

You can try this option from under windows to see if it will let you enter an UEFI options screen: Enter UEFI options 

So if you purchase one of these newer systems without a video card in it from let's say Amazon or some other location, and put in a non OEM card from another manufacturer, your chances are high you will run into this issue.

It will still boot into windows, but you will not be able to enter the bios because the bios uses a UEFI environment that requires an UEFI driver to enable the video card, and that driver will only work on the OEM cards and a select few aftermarket cards.

I have an RTX 3080 using DP port only,  and have no issues entering the BIOS menu on my R10. Latest Vbios is installed for the RTX 3080, and latest motherboard bios for the R10.

I believe it is related to DisplayPort connection to the monitor and something in the most recent update for the 3080 vbios. It appears that the drivers needed to send the signal to the display through DisplayPort cable are not loading until Windows loads. I have not found a solution to date other than connecting a monitor with an HDMI cable so I can get into the BIOs. I actually have a similar issue with a Lenovo laptop when it is connected to a dock with DisplayPort cables going to the monitors, so it is not exclusive to Alienware.