Default behavior for this only works if you are not an admin or SUDO user. If it is applying to non-admins/non-sudos, then you are correct and this could be a real issue. In other words, you will never see such a lock yourself. Also, are you sure they aren't clicking "Remove all Locks" in the menu?
There is such a setting: Lock Time-to-live (lock_ttl) can affect a such lock, by shortening the lifetime of the lock. When locking, a lot depends on timing. If they open the resource before you do, they will never see the lock, and because you are admin, neither will you. This results in the behavior you describe. Additionally, the lock may get extended by saving, etc, this can result in further confusing behavior.
There are two permissions that can be particularly troublesome, so you may have to check your ACLs. "remove_locks" simply allows a user to click it in the menu. "steal_locks" silently allows a user to steal the lock from other users. If this is set, they will never get notified of a lock. However, it still obeys priority, so a user can never "steal" the lock from an admin or sudo. This also results in the behavior you describe. They access a resource, but you have the lock. They have "steal_locks" so they are never notified, but the lock was never actually stolen.
Note: This is why on many systems, admins will move the resource to a different resource group while doing the editing and move it back when done. Since this is a common issue on many database systems, it might have been taken for granted.
Another Note: An sudo can steal a lock from an sudo, but neither will be notified, allowing both to make conflicting changes to the same resource. In these cases, the lock only locks out everybody else that isn't sudo.