Hmm, there is something I wonder:

when you label an email as spam it doesn't block receiving an email from the sender, but it moves any email into your spam folder. so you're still supposed to get it (in the spam folder)
Could it be something coming from bittrex mail server itself maybe?

if you have a slack account try to contact them in their channel, because with a support ticket it could be ignored with them :/

edit: tried to log in and received the email, so look like it doesn't come from bittrex