This problem only occurs (for me anyway) when the phone is laying on a flat surface, if it's in either of my hands then it's fine.
Strange that my last Z1 didn't have this issue, I read that lowering the pointer speed in language and input can help with the issue but it didn't for me, I suppose I'll just keep my phone in my hand if I need it.
To reproduce the error, place the phone on a surface (wood for example) then open your phone to the lock screen and use a circular motion but don't let go of the screen, if it unlocks then that's a break in contact.
Next replicate the issue while holding the phone in your hand, same circular motion on the lockscreen and it shouldn't unlock.
Make sure you don't have a case on at the time.
It's just a theory at the moment but my guess is that a particular material would be able to fix this issue if embalmed around the handset, so certain cases could actually help this issue.
I am also using a screen protector, just the one that came in the box, been put on perfectly, not a dent or bubble or blemish.
I'm running Monx's 4.3 rom with Doom kernel and my bootloader is unlocked, with a factory unlocked handset.