What kind of lizard? a gecko? it is pretty normal for them to hide in humid and dry hides.
I concur fully.Try making another soda bottle hide for the other side of the tank. I know some reptiles (I don't have leopard geckos but I keep snakes and used to have a crested gecko) are picky about hides. It might not be the humidity but the fact that she feels more secure in there. I had to get identical hides for my ball python otherwise she'd spend all her time on the side of the tank with her favorite cave.