Have you checked the settings on the App? Recent versions of iOS have upped the security on Apps. Go to Settings, and scroll down till you see the LibreLink App.on the first screen, make sure Bluetooth is switched on,and Mobile Data. On the same screen, click on the arrow for Notification. On that screen, Allow Notifications is switched on, and Critical Alerts as well. Make sure the Lock Screen, Notification Centre and Banners are ticked.
Further down, switch on Sounds and Badges, and if you wish Siri to announce alerts you can switch that on.
Once you’ve done all that, the settings will persist through any update on iOS. I’m running iOS 15.3 on a 12 Pro, though I’ve recently switched to self funding the Dexcom G6 with all the same settings switched on, apart from Siri, who gets on my nerves.
If it still doesn’t work after you’ve checked all those settings, you either need a new phone, or Libre need to get their act together to fix the App, though why your SE isn’t working or has a specific problem with iOS 15 is beyond me. Or maybe get Abbott to send you a reader, try and get out of paying for it because their app doesn’t work with your phone. Otherwise it makes a big hole in £200. Cheaper than a new phone, mind.