Hi there eebbrruu,
Welcome to Apple Support Communities. We understand you have a concern about not hearing notificaitons when your iPhone is locked. You also noted several items you've checked, which is a good start.
First, list out which specific apps you aren't hearing notifications for. Do you have the notification settings for those apps as a sound and a banner to appear on the Lock Screen? If so, do you not hear the notification but do see the expected banner or don't receive either?
When you checked to confirm your iOS is updated, did you also check for any app updates in the App Store? Also, go to Settings > General > Date & Time to confirm that is set correctly and "Set Automatically" is enabled.
Do you notice if the issue with notifications is specific to when you are connected to Wi-Fi or using cellular data? Do you use a VPN? If so, does disabling it momentarily help?
We look forward to hearing results.
Best regards.