I have scriptable cart ON and on sublist changed event each time I am getting transactionRecord.hasSubrecord('billingaddress')=false. but un Netsuite UI it give me true. Is this a bug?

I have not tried that specifically but scriptable cart does not behave exactly as it would in the User Interface of NetSuite. https://developers.suitecommerce.com/make-advanced-customizations-with-scriptable-cart.html

Tried getSubrecord() but it's empty in case of SCA transaction

I cannot say is that is possible since I have not used it in scriptable cart but compare what happens if you use 1.0 and 2.0 versions of suitescript. Probably your best bet to achieve your goal is creating an extension/custom module based on the version of SCA you are using.