I just spoke with DFAS. The amount are taxed and you must submit a form in the DFAS website in order to receive the rest on the amount that was withheld.
I separated on 28 Dec 18 and received my severance on 31 December 18. However, I was supposed to receive 136k and my check was only for 90k. Has your issue been resolved? I am currently talking to DFAS to find out where the difference is.