There is a forum member called Bevan Price who may be able to assist you with that query.
My books on the area published by Triangle Publishing are not exactly forthcoming with the exact date that you require.
I also have read some Triangle books about the Lancashire Union Railway and other Lancashire routes but you are correct in that it does not state when closed for goods, I have read elsewhere that Whelley stayed open for goods for many years without giving a date, but I do not know if Amberswood did?
I have walked the old trackbed a few times a few years ago and there were still signs of the platforms and a building was still extant that could have been the old station masters house, possibly turned into a goods office?
Thanks for the tip about Bevan Price, I will see if I can find him