What ever happened to Robb's decree concerning the heir to the North?

[drdoombot]

(spoilers for the very end of ASoS below)

At the end of ASoS, Jon Snow finds himself considering an offer from King Stannis to become Lord of Winterfell (while Samwell was conspiring to make him Lord Commander of the Night's Watch)... However, one plot point that seems to have been forgotten was that Robb had already legitimized Jon and made him King of the North!

In Catelyn V (just past the middle of the book), Robb meets with his bannermen not far from the Twins and states:

I've thought long and hard about who might follow me. I command you now as my true and loyal lords to fix your seals to this document as witnesses to my decision.

Earlier in the same chapter, Robb has a tense conversation with Catelyn about who should be his heir:

Jon is the only brother that remains to me. Should I die without issue, I want him to succeed me as King in the North.

This all transpires before the Red Wedding, so ravens were likely sent out with the seals of the lords of the North concerning Jon's legitimacy. Before King Stannis made his offer and the Night's Watch were voting for this new Lord Commander, Jon Snow was already Jon Stark and King of the North!

Why was this significant plot point never mentioned again? I can believe that King Stannis was withholding this information so that he could make his own (lesser) offer to Jon Snow, but I can't believe that Jon didn't find out through other means... surely a raven must have come to Maester Aemon or someone at the Night's Watch with Jon's legitimacy papers!

And now that he has become Lord Commander, what happens to Robb's decree? Can Jon abdicate his Lord Commandership to continue Robb's efforts to reclaim the North?

Is this at least mentioned in AFfC or ADwD? I would be very disappointed if this plot point was simply forgotten and never mentioned again. I think it would have been a great wrinkle to add to those last few chapters ASoS concerning Jon Snow's future.

[Maegor the Cuddly]

I imagine that some ravens got to their owners, but post probably did not. There's a whole "Great Northern Conspiracy" theory that, while it contains a lot of later spoilers, when you get to later on in the book you should look into. Stannis probably doesn't know, and even if he doesn't, Jon isn't King in the North, he abandoned all inheritance by taking the Black.