I just checked my Pi4 running OP3 & SK shows the same "alertCategory", "alertSystem" and "alertID" metadata as the Pi5 OP4 and that is using the "Simple Notifications" App.
I've never had a pop-up notification using OP3 either - not since OP2.
I've never had a pop-up notification using OP3 either - not since OP2.
Pi5 8GB, NVME SSD 512GB, MacArthur hat, OP4,