I've given Elise panadol quite a few times now and I haven't had a stain from it yet on any nappy.
I think it might be a case of "if the conditions are right" then you will get a stain.
Maybe it's if they have had it more than once in a certain time, the acidity of their wee, I dunno, but I've not got a stain from it yet and there have been days where I've given it to her a few times.