On the FP4 notification sounds for individual apps can’t be set to “None”, compare those two screenshots (both Android 11) and the missing option at the top:
Setting it “None” (“Ohne”) on the Fairphone will actually set it to “Default” (“Standard-Benachrichtigungston”). I’d call that a regression (albeit a low impact one) that definitely should be tracked.
Setting it to “None” globally still works though.