Séb, I can confirm what Sam and coeur-noir are seeing. No icon is displayed on notifications with that example website you linked to, and using dbus-monitor I can see that the icons are being saved under the snap's confined TMPDIR, so the path is not visible from the host.
This would probably need a patch to rewrite the path, or save the temporary icon file to a different place that's visible to the host.
Séb, I can confirm what Sam and coeur-noir are seeing. No icon is displayed on notifications with that example website you linked to, and using dbus-monitor I can see that the icons are being saved under the snap's confined TMPDIR, so the path is not visible from the host.
This would probably need a patch to rewrite the path, or save the temporary icon file to a different place that's visible to the host.