After order, assigned to a second user group

Hi,

If you don't use the "user group after purchase" setting, disable the HikaShop group plugin via the Joomla plugins manager.
That way, you'll be able to ascertain whether it comes from that plugin or not.

It could also come from a mass action, so go in the menu System>Mass actions, and make sure that the mass actions are either disabled or removed if you have any.

Otherwise, it's hard to say what it could be. I think that you should first determine when that happens exactly. Does that happen at the end of the checkout, before the customer is redirected to the payment gateway, or after the payment is made on the payment gateway, or when the user registers on the website ?
Then determining a way to reproduce the issue 100% of the times would be great. That might already help understand what triggers the problem. It's likely to come from one of the hikashop or system group plugins, so deactivating them one by one and testing would allow to narrow it down to one plugin which will make it a lot easier to understand what's going.