I just updated the kernel on my Ubuntu 20.04 LTS to 5.8-0-34-generic. Now my Ubuntu no longer opens full screen, opening with 800 x 600 resolution rather than the 1920x975 pixels available on my display, with no option to increase screen resolution. Fortunately I had a clone still running the 5.4.0-59-generic kernel, which continues to work correctly.
I'm running Ubuntu 20.04 LTS in a VirtualBox (Version 6.1.16 r140961 Qt5.6.2) virtual machine, original Linux kernel 5.4.0-59-generic, upgraded to new kernel 5.8-0-34-generic. The system graphics card is an NVIDIA GeForce MX 330. Further exploration reveals that the problem probably results from an incompatibility of the virtualbox-guest-dkms 6.1.10-dfsg-1~ubuntu1.20.04.1 package. I have submitted a problem report to debian.org, which appears to be the source of the package, and I have submitted the problem to the VirtualBox community help site.
I'll follow up here if I get a response from VirtualBox community help.