@AngelK 

I don't think it's a controller's config reason since the Androids works correctly.

Please make sure if the notification is enabled on the iPhones, it should be on System Settings > Notifications.

  @Vincent-TP 

Of course, the first step in troubleshooting was to check the iPhone settings.

The app was also suspected as a possible cause, but since the issue first appeared, there have already been three updates. 
I have also uninstalled and completely reinstalled the app, without noticing any difference from before.

Additionally, no similar reports have surfaced in relevant forums, which leads me to believe that this is a local issue.
The only change that comes to mind is the OC200 update.

As my next step, I plan to run the LAN ports on my switch in mirror mode to analyze the network traffic and determine where the data is going when the doorbell rings.
Fortunately, I can also capture outbound traffic on the WAN side, allowing me to identify any potential differences.

A packet capture directly from Omada, without the detour via port mirroring, additional hardware for capturing, etc., would be highly beneficial for troubleshooting.
AVM has had this functionality built into the Fritz!Box for many years now…