If you contact Apple, they maybe able to help you and if Apple has a fix, you can pass on your fix from Apple to these community support forums for others to help them fix this smart cover issue.
I already advised some of these users, with this smart cover issue, to contact Apple support, as well.
I have discussed this smart cover issue in another private Apple community forum with my fellow, long-time Apple Support Community contributing users about this iOS 12 smart cover bug/issue.
They all have the smart cover Lock/Unlock Settings option.
None of us know why some iPads that get upgraded to iOS 12 are missing long-time features.
Some users are reporting that that with new iPad Pro models, that the Tru-tone option is not appearing.
Other iPad users are reporting that the FaceTime front facing selfie camera option is missing, also.
It would appear that the initial release of iOS 12, much like iOS 11, is still very buggy or some features have been accidentally or arbitrarily omitted.
Nothing we users can do.
This is one of the reasons I hold out to update my iDevices until the point releases start making their appearance.
I won't upgrade my iDevices to iOS 12 until, at least iOS 12.2 or iOS 12.3.
Since you cannot return to a previous iOS version once you upgrade/update to a new iOS version, I always wait it out.
I never upgrade my iDevices at very first introduction of any iOS version.
AND iOS 11 was sooo bad, I skipped iOS 11, entirely.
So, I will continue to wait out iOS 12 until all the obvious major bugs/issues get resolved.
Once again, contact Apple and Good Luck to You!