Not sure how the hacker has done it without
1) 2FA being enabled on your account
2) Email verification on yobit's side
3) Login notification not being sent to your email
The only explanation i can think of is that all of your accounts have been compromised, your email, google authenticator, etc. Or it was one of your family members doing it that has physical access to your computer and mobile phone.
Just a heads up though, in these scenarios yobit can't do anything. Even if you contact support they won't be able to resolve the issue, the only trail the hacker has left is the yobicode. You may be able to ask the support to verify which account redeemed the yobicodes(even though it's probably a throaway).
I think not only hacker is suspected here that honestly hard to think if how the hacker got the 2fa of his account unless if he is not adding 2fa authenticator.. I never heard this issue before in yobit but the other suspected here is the yobit because no one couldn't know the 2fa code except for yobit.
Or maybe the other way is someone install teamviewer in his computer to remotely use the google authenticator..
There are lots of ways honestly but if you didn't receive any notification that someone logging to your account (If the ip changes in my case before i receive notification) you should receive notification unless if you are using the same computer and ip..
Better to keep trying to contact yobit and ask for the ip and time listed in their system logs so that you can review this problem. .