I hate hearing things like that.
It is very possible that DFAS has not updated whatever system they are using to calculate severance pay. I would call DFAS direct and see what they say. I would hope that they have some method to manually override.
However, if they do not process your pay correctly, you have the old standby Board for Correction of Military Records to appeal for relief. The thing that disturbs me about this is that the relief granted would be to direct DFAS to pay you. This should be the same outcome that comes out of someone at DFAS doing the input right the first time (though, there is a code that DFAS uses for paying out corrected amounts through the BCMRs- I really hope that someone has enough sense to figure out how to process this without making you get a finding from the
BCMR).
If it turns out that you cannot get this straightened out with DFAS, please let me know ASAP. If you are agreeable, I will push this issue. This is not acceptable.