Icons for Chrome web apps won’t show

$ lsb_release -rd
Description: Ubuntu 20.04.1 LTS
Release: 20.04

pcmanfm-qt 0.14.1-0ubuntu3

Chrome web apps are supposed to show their custom icon in file manager and menu, like the google drive logo or the google sheet icon, and that's exactly what happens on another system with kubuntu 20.04

Instead, only the main chrome icon is shown.

some interesting notes:

Fields is correctly set on the .desktop file.
here’s an example

#!/usr/bin/env xdg-open
[Desktop Entry]
Version=1.0
Terminal=false
Type=Application
Name=Google Drive
Exec=/opt/google/chrome/google-chrome --profile-directory=Default --app-id=aghbiahbpaijignceidepookljebhfak
Icon=chrome-aghbiahbpaijignceidepookljebhfak-Default
StartupWMClass=crx_aghbiahbpaijignceidepookljebhfak

to make the icon show I had to correct the line with

Icon=/home/USER/.local/share/icons/hicolor/48x48/apps/chrome-aghbiahbpaijignceidepookljebhfak-Default

Icons should be registered with a vendor name prefix, like "numix-monitor", but I noticed that if I install an icon with the chrome prefix, it won’t show: only the default chrome icon will appear

The directory where systems mostly look for user icons is ~/.local/share/icons , especially the "hicolor" theme which acts as sort of default directory.
This seems to work perfectly on my system: if I install a custom icon with the command

xdg-icon-resource install --size 256 prefix-icon.png

everything works smoothly: even if I may need to reboot, the icon file appears on the directory above, just like the correctly named icons for chrome web apps are already in the directories they should be, and using “prefix-icon” in the icon field of a .desktop file will show that specific icon on desktop, in file manager and in the apps menu.

Installing the icon with the --novendor won’t help, the length or the suffix or the icon name won’t change the result