Default Alert Tone randomly plays on iPhone (iOS 26)

My iPhone 15 Pro (iOS 26) makes the Default Alert sound in this case "Slide"; however, there is no new notification when I unlock my phone. Not even the screen flashes when the sound is played. I have the notification settings set to confirm that if any app is only sending only sound notifications, but there is no such app. Even my YouTube and Mail notifications are weird. YouTube notifications have no sound at all. Only banners. For the Mail app, it does deliver me my mail notifications, but like a day later, they all come at once. I'm fed up with this system. Screenshots for both are also attached.