Has anyone experienced a primary currency change on the customer when the customer logins to webstore?

What exactly do you mean? If they select a currency before they log in, then NetSuite will switch to the currency on their customer record

The possibility to change currency is at least not visual. I haven't seen it before. It get's changed on every login.

As a rule, we don't allow currency changes once as user has created an order, so I'm still not entirely sure what behaviour you're describing

It happens for some customers only.. it is very strange. In system info on customer record, it has no context and the customer's name is the editor

Is this a Lead or a customer? If this is a customer (placed at least one sales order), this is not possible, even in the ERP, you cannot do that. If this is a Lead, chances are you have something (code) updating the currency.

It is a customer. It has not placed an order. But currency can be changed after sales order has been placed in ERP.

So, in NetSuite terms, it is a Lead, not a customer. If the currency can be changed, it might be due that the subsidiary assigned to the Lead/customer allow those currencies.

No, it is a customer in netsuite terms. And yes, the currencies are allowed. But the change is happening as soon as the customer login. That's all he does, simply login and the currency changes.