I am running fedora 33 and I noticed that the grub menu has added a “System Setup” option at the bottom of the list of kernels to choose. Yes, I like having the grub menu displayed even on a single OS machine. 
Just to see what it did I tried and found that it, as expected, opens the UEFI bios. Since I was happy with the way everything was configured I made no changes and exited the bios with my normal F10 - “exit and save” and rebooted.
To my surprise I found I could no longer see the bios splash screen nor the grub menu but it booted directly into fedora. Since I wanted the short pauses so I could select entering bios, the bios boot menu (needed to boot to USB), or choose the kernel to boot but could not even open the bios during boot I finally opened the machine, drained CMOS so the bios was back to default, then was able to finally boot in my normal method after resetting the bios to support my hardware.
I then went hunting to find what selecting the “system setup” option in Fedora’s grub menu had changed to bypass both the bios screen and the grub menu and found within the bios a “Fast Boot” option whose description fit what had happened. “Fast Boot” bypasses the bios splash screen, disables the ability to enter bios, and instantly loads the primary OS on the system. It also warns you that the only way to reset it is to drain the CMOS.
Apparently selecting that boot option in the grub menu takes you into the bios and one of the hidden changes it does is set the “Fast Boot”. This is totally unacceptable for an innocuous, seemingly normal, boot option to do this without informing the user of this change being done.
I do not necessarily think that is bad by itself but to have what appears to be a normal boot into bios option then make changes the user is unaware of is (to put it mildly) very irritating. To then have the only method of recovery be opening the case of the machine and short the jumper to drain cmos is likely far beyond the capabilities and comfort zone of the average user.
While I am not personally filing a bug against this, I am aware now of what it does and will never use it again.
I also wanted to let others know of this hidden “feature” that goes totally against the ability to even use the shift key and get the grub menu to display during boot. It also disables access to the bios boot menu. For those that seem to be unable to access the grub menu during booting when trying to access an older kernel this could easily be the cause, and it may be required for them to drain the cmos and then reset the bios options for their hardware before they can even get to the grub menu.