For a while now, I’ve been missing some notifications, beeps, and other things from my phone. Turns out it wasn’t just me, but a new iOS setting called “Attention Aware” See Turn Attention Aware features on or off - Apple Support
Frankly, this really pisses me off. Apple should NOT by default change the behavior of the device without telling the user “here’s this new thing and it impacts how you interact with your phone. Should I turn it on?”
I believe it’s not new - I think it goes back to the iPhone X.
See this 5 year old Reddit post, which describes the feature as it exists today: https://www.reddit.com/r/iPhoneX/s/ZE5yOtrouX
This is definitely not new, iOS has been doing this for years, and what it does is make the notifications gentler when you are looking at your phone. Since you’re looking at the screen at the time, it should be pretty hard to miss the notifications. Perhaps what you’re experiencing is a Focus mode that’s turned on?