Anyone could actually fix the notification bug? (I'm one of those who randomly doesn't receive notifications)

Yeah, many posts here about this, a lot of complaints also in other communities. Of course, I've disabled "optimization", allow background activity, and so on. The OP12 is my fifth device by OnePlus (OP3, OP6, OP8T, OP Tablet), so I thought I know what to do. My version is OxygenOS 15.0 CPH2581_15.0.0.503(EX01) ... FYI, there is this website as well https://dontkillmyapp.com/oneplus

But still, several apps (Nine mail, Telegram, Discord, Teams, Amazon, and probably many more) won't show notifications. Sometimes they are delayed, sometimes they come up when I open the app, sometimes they are not appearing at all (I open the app and see there are new messages, but no notification).

I'm trying for two weeks now to fix that, but I'm really exhausted. I have still a few days until I can return it and as some other (minor) things also annoy me, I'm going to do that if the notification shit isn't going to work.

Any ideas for 2025? Any final solution to this issue? If not: any recommendation for a good phone?