This is the official reply to my AMEX Inbox:
Dear angeloedades,
I see that you received a credit of £5 on your account as per the offer from American Express. However, our system considered it as a merchant credit and 8 Avios were reversed from your account.
Our Rewards Team identified this error and applied 8 Avios back to your account.
If there is any other way in which we may be of assistance, do not hesitate to contact us. We work round the clock and would be happy to help you here at American Express!
Sincerely,
XXXXXXX
/AE