Amazon Sponsored Ads Fraud?

Hi,
I received an invoice for product ads on French Amazon for May.
It was about 8-10 times more that we paid on average for the same campaigns every month.
In May alone for every product sold (worth 17-18 EURO) we spent 5 Euro on sponsored ads which is 6-7 times more than in any month over the last year!
Amazon keeps insisting that everything is fine and in last email claims that it is a sign of a “successful campaign”.
Well 5 Euro for product sold plus FBA charge for one product makes it below profit. Very successful.
Upon further investigation we checked other months. Similar things happened in January and April only this time it was 3 and 2 Euro for each product sold.
We cancelled all campaingns on French Amazon. There is clearly something wrong there.
Could someone explain this? Anyone had a similar situation? I hit the wall at Amazon support. They claim there’s nothing wrong and refuse to investigate further.
Please check your ads invoices/reports from France and let me know if something similar happened to anyone since January.

Kind regards

Could be competitors, could simply be customers who decide they don’t want that.
No way of knowing.