Hey PezTL,
I see that you're having trouble locking your iPhone 8 Plus after updating. I'll be happy to help out.
Since the issue started occurring after updating the device, you may want to reinstall iOS to see if it resolves the issue for you. Before you proceed, please back up your data using the steps below. Reinstalling iOS only overwrites iOS on the device, and does not affect personal data, but it’s always recommended to have a backup first: How to back up your iPhone, iPad, and iPod touch.
Once you've verified that your backup is current, follow the steps below to reinstall iOS on your iPhone. Keep in mind that this article was written for a different issue, but these steps will still be helpful. Also be aware, you'll want to click the "Update" button and not the "Restore" button when prompted as the "Restore" button will restore the device to factory settings: If your iPhone, iPad, or iPod touch won't turn on or is frozen.
If the Side button is still not working, see this: If a button or switch isn't working on your iPhone or iPad.
Have a great day.