Does the NTSB ever not follow up a preliminary report?
As a retired NTSB employee, I can say that reports with an FA number like that one should definitely close out with a probable cause determination, typically within a year.  Two years is an outlier, and the delay is likely attracting some internal attention.  If you really want to find out about the status, you can call 202-314-6290 and ask for the Ashburn field office.  It's probably their case, and the manager should be able to figure out what's holding it up.