I don't know if the lead/silver mix was different.

The paper specifically calls out mining activity around the Rhine as having significant impact on the lead signal, with a 2-5 fold greater impact compared to Spanish sites. The authors suggest that the 700s spike in lead pollution was driven by increased activity in France and Britain (which once again has a many fold greater impact on this measurement than in Spain).

Also, doing a cursory look at sources suggests that the eastern empire relied far more on gold and bronze coinage than silver, suggesting another possible source for the lack of an Eastern Empire signature.