I understand you're having some trouble with the Auto-Lock on your iPhone. I would want my display responding as expected, so let's try and find a fix together.
First, let's force close any open apps, and restart your iPhone. Here's how:
How to force an app to close on your iPhone, iPad, or iPod touch - Apple Support
Restart your iPhone, iPad, or iPod touch - Apple Support
After your iPhone is through restarting, let's go to Settings > Display & Brightness > Auto-Lock.
Set the timer to 30 seconds and then let's wait for the timer to run out and lock the screen. If that works, set it back to 1 Minute and test again.
If that doesn't fix the problem, make sure your iPhone 8 is backed up and up to date:
Update the iOS on your iPhone, iPad, or iPod touch - Apple Support
Let me know what happens.
Thanks for using the Apple Support Communities.