If Coinbase can cancel your order if the price goes up, but you can't cancel an order if the price goes down, then what they are doing is essentially stealing a free put option from you.
It is my assertion that this is exactly what they are doing and it is fraud.
I keep seeing the same thing over and over. People having orders canceled as the price rises while orders at a higher price go though. They complain publicly, their issue is fixed and they act as if everything is now fine.
It's not fine.
Someone please explain to me what kind of fraud detection algorithm stops nearly identical transactions from the same person. Their "fraud detection" looks a lot like fraud perpetration.