Hmm, I'd suspect that the pylon is to blame here and the boss "cut off" check doesn't correctly count those as non-usable squares. And what do you mean the cross disappears? Like completely vanishes and makes you unable to continue?
I should probably say invisible cross, only the sparkles behind are visible. It happens rarely and to me for no discernable reason. I posted it some time ago, I'll try to find that screen
Of wow, that's weird. I guess I've completely missed (or somehow forgotten) that post. Does the cross stay invisible for the whole level or does it phase in/out?