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We had a 16 minute battle in GvG and at the end when we killed the Guild Lord (right at the 2 minute res mark) it said "Guild Battle Ended Prematurely" and sent us back to the hall. Neither us nor the other team got credit for our win.
It happened to us 3 times now
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Here's two screenshots of the two battles we won, yet didn't.
EDIT- BlackArrow:
We fought your guild who put up a good fight might I add, the first GvG. Then we fought DA and had some miscommunication in our team leaving us split up as they assassinated our Guild Lord. Then we fought your guild again in our third attempt at GvG (and last*) for the night.
*because it didn't give us credit for wins/losses--good practice though.