The white line can happen on lots of other fabrics, not just batiks.
Look very closely at your other fabrics that you have used it on. If they were light, there is probably a ghost line there, but is faint and thus why you do not see it. I've even seen it on white-on-white fabrics .... but yes, for the better part you do not see it.
As you now recognize ... testing before use, is important, to avoid problems!