Granted, I haven’t looked closely at a Russian passport for the last 2-3 years, but when I did, it mentioned nationality. This was a Russian passport. Not Soviet.
http://en.m.wikipedia.org/wiki/Internal_Passport_of_Russia
In fact a post-1997 form lacks blank space to put ethnicity into.
You have probably seen older-issued passports. I might be wrong if you are so sure but AFAIK it was an older Soviet form.
BTW, there is a reform in place to abolish internal passports and use driver’s license or sosec ids instead.