Not possible to set 'Alt+PrtScr' as shortcut key for 'Active Window' in ScreenGrab

Release: Lubuntu 21.04

Software, version: screengrab, 2.1.0-1ubuntu1

How to reproduce:
1) Boot into Lubuntu desktop.
2) Start 'ScreenGrab' application from 'Graphics' menu.
3) Select 'Options' from the toolbar, navigate to 'Shortcuts'.
4) Under 'Global Shortcuts' select 'Active Window' and double-click. 'Selected Shortcut' will get activated, waiting for our input.
5) Try to set 'Alt+Print'.

Expected:
Able to set shortcut.

Actual:
Not possible to set 'Alt+Print' as shortcut. It will show 'Alt+...' but once we release keys, it stays as '...' waiting for keys input.

We can set other key-combinations as shortcut, including ones involving 'Alt' key, but not 'Alt+Print'.

No issue setting keyboard shortcut to 'Full screen' or 'Area select'.

Note:
This issue was also observed on 20.04 with screengrab version '2.0.0-1'.

ProblemType: Bug
DistroRelease: Ubuntu 21.04
Package: screengrab 2.1.0-1ubuntu1
ProcVersionSignature: Ubuntu 5.11.0-16.17-generic 5.11.12
Uname: Linux 5.11.0-16-generic x86_64
ApportVersion: 2.20.11-0ubuntu65
Architecture: amd64
CasperMD5CheckResult: unknown
CurrentDesktop: LXQt
Date: Sat Apr 24 19:16:10 2021
InstallationDate: Installed on 2021-04-22 (2 days ago)
InstallationMedia: Lubuntu 21.04 "Hirsute Hippo" - Release amd64 (20210420)
SourcePackage: screengrab
UpgradeStatus: No upgrade log present (probably fresh install)