update-grub is not detecting prior QA-test install (likely b/c BTRFS)

lubuntu 20.04.3 QA-test install on
- hp dc7700 (c2d-e6320, 5gb, nvidia quadro nvs 290)

** Summary

update-grub is not detecting prior QA-test install; grub is only offering new install as boot option

Two installs were performed on this box
(1) Manual partition; two partitions / (btrfs) & /home (ext4)
(2) Install alongside; the /home was selected to be split

** Expected Results

I expect a new install to show two choices in grub
- (L)ubuntu 20.04.3 (first install)
- (L)ubuntu 20.04.3 (this new install which was an 'install alongside'

** Actual Results

First login (of new install) and I ran `sudo update-grub` (after copying first grub.cfg to grub.cfg.org) and `diff` shows only

guiverc@dc7700:/boot/grub$ diff grub.cfg grub.cfg.org
103c103
< set lang=en_AU
---
> set lang=C

so one first boot into new system, I get the same result as it did during install. (the `diff` results could be common, or a result of me not having the ethernet cable in box at time calamares was started for first install; prior install was no-internet so cable had been removed)

Using `pcmanfm-qt` to mount partitions and re-run `update-grub` & `diff` with & same answers.

** Likely Cause

The prior install was a BRTFS "/" partition which is likely cause. It likely won't impact many users, in fact when navigating there in `pcmanfm-qt` I get a "@" directory before I see the actual real "/"

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: grub2-common 2.04-1ubuntu26.12
ProcVersionSignature: Ubuntu 5.11.0-25.27~20.04.1-generic 5.11.22
Uname: Linux 5.11.0-25-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.18
Architecture: amd64
CasperMD5CheckResult: skip
Date: Tue Aug 10 15:44:18 2021
InstallationDate: Installed on 2021-08-10 (0 days ago)
InstallationMedia: Lubuntu 20.04.3 LTS "Focal Fossa" - Release amd64 (20210809)
SourcePackage: grub2
UpgradeStatus: No upgrade log present (probably fresh install)