Assigning SnapChat to my camera button shows a locked, black screen on iPhone

Here’s the steps I did:

Settings > Camera > Camera Control > Snapchat > turned off “Require screen on”

So I turned my phone off, clicked my camera button, and it showed me a completely black screen with a lock icon at the top. So I turned my phone off, turned it back on to show my lock screen, clicked the camera button and only then it worked. But I shouldn’t have to turn on my power button to make the camera button work if I already changed those settings.

I tried everything I possibly could to troubleshoot already. Restarted the phone, checked for phone updates, checked for app updates, uninstalled Snapchat and reinstalled, etc. I even tested it with Instagram and the same exact thing happens. Why is this an issue?

[Re-Titled by Moderator]