lubuntu hirsute QA-test install & prior ubuntu system fails to boot

Lubuntu hirsute (daily) QA-test install on
- hp dc7700 (c2d-e6320, 5gb, nvidia quadro nvs 290)

Testcase: install alongside

Note: Lubuntu should be using the `calamares` installed, but due to https://bugs.launchpad.net/ubuntu/+source/casper/+bug/1923881 the current ISO is using `ubiquity` instead

Prior to install there was one partition (sda1) which contained Ubuntu hirsute (a prior QA-test install).

It was tested & booted normally prior to this QA-test

** expected result

Post-Lubuntu install; I have two systems

- ubuntu hirsute (boots & operates as it did prior to this QA-test install)
- lubuntu hirsute install

Both available via grub

** actual result

- ubuntu fails to boot; recovery console only
- lubuntu install works perfectly.

** changes noticed

From a single partition (sda1 using all disk space) I now have

sda1 ext4
sda2 fat32 /boot/efi in lubuntu
sda3 extended
sda5 ext4 / for lubuntu

I wonder if the addition of the fat32 has altered the booting of ubuntu hirsute.

I'd really like to search the iso.qa.ubu test results & find my last ubuntu hirsute install to confirm how I recorded the tests.... When I booted ubuntu; I expected it to fail to boot (been having loads of failures lately with Ubuntu hirsute on BIOS boxes) but it booted correctly, looked good so I expected the same post this install... I'm wondering if the successful boot was just a fluke (but that's likely worse!)

This whole bug report maybe pointless due to aforementioned Lubuntu using `ubiquity` to install issue.. but I've reported in case it shows an issue with Ubuntu ubiquity (and BIOS/non-EFI install where ESP partition is added) on pre-EFI boxes, and I need a bug report to fail the iso.qa.ubuntu.com result..
-

ProblemType: Bug
DistroRelease: Ubuntu 21.04
Package: ubiquity (not installed)
ProcVersionSignature: Ubuntu 5.11.0-14.15-generic 5.11.12
Uname: Linux 5.11.0-14-generic x86_64
ApportVersion: 2.20.11-0ubuntu62
Architecture: amd64
CasperMD5CheckResult: pass
CurrentDesktop: LXQt
Date: Thu Apr 15 12:40:48 2021
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/lubuntu.seed quiet splash ---
InstallationDate: Installed on 2021-04-15 (0 days ago)
InstallationMedia: Lubuntu 21.04 "Hirsute Hippo" - Beta amd64 (20210414)
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)