What I was looking for is the news to prove that Bitmain sold off Hashnet at some point, as you stated in the OP.
Someone has asked about it. No proof was provided but I don't think they would lie about this since they were one of hashnest's victims.
I have already written to bitmain repeatedly and they deny they have relationship with hashnest. They say they have "separated" from hashnest, that they are different entities, that I am complaining with the wrong company and that they can't help me.
Strangely, only one news outlet has mentioned it, which is South China Morning Post
...Hashnest, a cloud-based mining platform, is no longer a Bitmain subsidiary but an external project funded by it.
A month after that, there was an interview from Hashnest CEO where it he implied the parting ways
CAILU: Hashnest was founded in 2013 and is a cloudmining platform under Bitmain. Can you briefly tell us the birth of Hashnest and your plans for it?
ROB: Hashnest was initially known as Snowball.io and was nothing more than an exchange. It was only later it became a cloudmining company. In 2014 I got to know Wu Jihan, and the company was officially bought over by Bitmain in July. We rebranded it to “Hashnest” and swiftly it was launched in September.
It has been over 6 years since the creation of this company. Our users have also been rapidly increasing; we have over 320K users now!
This year, we decided to become independent from Bitmain....
Overall, I believe this should be enough to back OPs claim.