I am not a lawyer, but I would expect that having officially known it was not valid in June but choosing not to take any action to remedy it before attempting to use it again in Sept/Oct might disqualify you from claiming any fare increase during that new 4 month later booking period.

You might be lucky of course, and do of course go for it if you think you're in the right. But having sat on it, and then also received the same voucher value discount via the social media team, may mean it's calmer to just move on.

PS - And please try to stop using the word fake. It is not fake, despite the cynical misuse of that word by certain people. The voucher does not work for some as yet unknown reason but BA have accepted it has value and have given it to you for your subsequent booking. Trying to claim some kind of fraud having received the benefit anyway won't be helping you. Just trying to help