I recently faced similar (yet not the same) issue with Foma emulsion: I double coated watercolour paper, exposed contact prints on it - while in developer or the in a stop bath, the image looked quite well, greys and black were even; once put into fixer, brush strokes and paper texture became very visible. And yes, the picture got a bit darker (not blackened, anyway), but that's expected.

I wonder if triple (or more) coating may help. The paper is quite coarse.

I use acidic stop bath, also because the watercolour paper seems to be pH-balanced using some alkali - it hisses when poured into acidic bath. Next time I'll buy better one... Anyway, if it's not enough stopping in water that causes blackening, you may try acidic.

Gandolfi: the two students of you, did they both expose and develop the prints the same way?