I was having the same issue with the device also locking if I had used a gesture (double tap in nova launcher with the Screen Off and Lock app) to turn the screen off. Smart lock would only work if I had used the power button. However, I just stumbled on this obscure looking app that is able to mimick the power-button, which has fixed the issue for me. ROOT is needed for this!
Here what you need to do:
Download an app called Power Button from the Play Store (I can't post the link, but it's by Torcellite) and open it.
In the quick setup grant it superuser rights and in the next step let it automatically map your power-button. That is all, and you can close the app now.
When you choose a gesture, make sure you assign it to the Screen Off shortcut and not the app itself (in nova this is the most right tab).
Final note: if you have the Power Button app opened (as a window in the background), using the gesture will bring it to the foreground and it will show up when you 'unlock' your screen the next time. But fortunately there is no need to ever open the app again after the initial setup, so don't worry about this.