upgrade modifies /etc/default/grub inappropriately

[Allan Dyer]

If I have:
GRUB_CMDLINE_LINUX="nomodeset"
in /etc/default/grub, boot hangs with "Failed to start Light Display Manager". No problem, I delete the line and run update-grub.

However, running
apt upgrade
sometimes appends the line
GRUB_CMDLINE_LINUX="nomodeset"
to /etc/default/grub, resulting in a failed boot after upgrade. I am sure it is appending the line to the file because the rest of the file is unchanged.
This has been happening occasionally for a long time. Most recently, it happened when I used
do-release-upgrade
to go from 18.04 to 20.04
I guess some package thinks it is a good idea to add this option, but I don't know which one, or how to stop it.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: apt 2.0.2ubuntu0.1
ProcVersionSignature: Ubuntu 5.4.0-52.57-generic 5.4.65
Uname: Linux 5.4.0-52-generic x86_64
NonfreeKernelModules: talpa_vfshook talpa_pedconnector talpa_pedevice talpa_vcdevice talpa_core talpa_linux talpa_syscallhook
ApportVersion: 2.20.11-0ubuntu27.10
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Wed Oct 28 16:51:45 2020
InstallationDate: Installed on 2017-06-29 (1217 days ago)
InstallationMedia: Ubuntu 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160719)
SourcePackage: apt
UpgradeStatus: Upgraded to focal on 2020-10-24 (4 days ago)