Grand eldritch Ember can't roll vulnerabilitycurse effect on helmet

Grand eldritch Ember can't roll vulnerability curse effect on helmet.

I spend 500+ Grand eldritch Embers, I saw every single mod multiple times.
I saw every single curse effect 5+ times.

I haven't saw Vulnerability curse effect over 500 rerolls.

Could you take a look on it and see if it's a bug/weight bug/or I just extremely unlucky until I spend another 500 without result pls.

Best regards.

RNG