Hi,
CustomerService.getCustomers() does not work as documented.
Old forums threads were quoting the docs, but I find more recent threads saying that without a customer ID header the method behaves like the old CustomerService.get (returning the main account for the authenticated user), and that it currently always returns a single account is the list.
Please please fix this, I probably won't be the last one to fall into this trap.
Besides, the SOAP API reference and the developer guide does not seem to agree, the former says that the behaviour changed since v201605, the latter says it's since v201607: