I recently made a booking through Accor's website for a Junior Suite at one of their properties using Suite Night Upgrades (SNU) I received as a platinum member for the first time. I followed the instructions and everything was working and booking was confirmed with the SNU. As it was an advance rate I paid the full amount for the stay during the booking process and the full amount was taken from my credit card. In the confirmation it showed '0 Euro' as the amount to be paid at the hotel.
A week later I noticed in my ALL account that my SNU were back, i.e. I could still use them or use them again. And my booking suddenly showed over 300 Euros still to be paid at the hotel. Not wanting to pay hundreds of Euros extra on arrival, when I already paid, I raised this with Accor, but was told I owed this money and should pay this as soon as possible; they cannot apply SNU to an existing booking
Something must have gone wrong on their side and I offered to send them proof of my SNU booking. But they neither seem interested in that or resolving their error.

Has anyone else had problems using the SNU? Is this normal and a way for them to get extra money?

I was told I can still use the SNU for another booking, but that would mean even more money I need to spend on a stay I don't want. The SNU feel more like a scam to get you spending extra money than a reward for being a loyal customer. I even tried their dedicated customer care number for platinum members, but that only goes to an outsourced call centre in Asia, with pretty much the same response you get through their contact form.

Any suggestions for next steps, when they are not even offering the cancellation of the booking? Claim through my credit card provider?