Applications not showing custom icons in the Grouped Window List or Alt-Tab app switcher in Mint 20.3

 * Cinnamon version: Cinnamon 5.6.7
 * Distribution: Linux Mint 21.1 x86_64 
 * Graphics hardware *and* driver used: AMD ATI Radeon RX 6700/6700 XT / 6800M, driver=amdgpu
 * 32 or 64 bit: 64 bit

Issue

Some applications added in the Cinnamon Menu Editor do not display the expected icons in the Panel (grouped window list applet) nor in the Alt-Tab application switcher.

Steps to reproduce

Create a new menu shortcut with the Menu Editor tool (cinnamon-menu-editor), or write a .desktop file manually and place it in ~/.local/share/applications. For my example, the icon is a .png located in ~/.icons, without a subfolder.
The relevant line from my application's .desktop file is:
Icon=alephone.png

Expected behaviour

When the application launches, the icon specified in the .desktop file should be shown not only in the Cinnamon menu applet, but also on the Cinnamon Panel and in the Alt-Tab app-switcher menu.

Other information

The full contents of the file ~/.local/share/applications/alephone.desktop:

#!/usr/bin/env xdg-open

[Desktop Entry]
Name=Aleph One
Exec=/usr/local/bin/alephone
Comment=
Terminal=false
Icon=alephone.png
Type=Application
Name[en_US]=alephone.desktop
Hidden=true

Configuration info, if applicable

I have uploaded the settings for grouped-window-list@cinnamon.org:

Screenshot of all relevant information:

Start by reproducing the issue with the provided .desktop file and compare the grouped window list applet with the Alt-Tab app switcher. Trace how each component resolves the Icon value for applications. Done means custom application icons appear consistently in both the Panel and Alt-Tab switcher.