hello, welcome.
This is because you are trading CFDs, but I doubt it's because the CFD provider is doing something nefarious.
This is what I think happened:
* You entered the trade and put a stop at 6.28
* Your assumption is that the stop is triggered when a trade in the underlying market occurs at or below 6.28 during market open hours 10AM-4PM
* In reality, their stoploss logic for buys executes the stop when the bid on the spread of the CFD exceeds 6.28 at any time
* At some point, probably in pre or post market auction, the spread widened and they tried to dump their hedge of your position into the underlying market, when the book was thin and very wide.
The stoploss logic is optimised for 23/6 markets like forex.
CFD stoploss works on the spread not trades in the underlying, that is the price you pay for not trading in the underlying market (crossing the spread to trade in return for 0 commission).