Quick answer: Stops execute on the price you can actually trade at — which includes spread — not the mid or chart price. Around news and rollover the spread widens sharply, so a stop can fill while the chart never appears to touch it.
What actually happens
Buy positions close on the bid, sell positions on the ask. Charts usually draw the bid, so a sell's stop can be hit by the ask while the chart line never reaches it
Spread widening during news, low liquidity and the 5pm EDT rollover moves the tradable price away from the chart price
Slippage can fill a stop beyond its level in fast markets
Rollover is the classic case
The daily reset at 5pm EDT coincides with a one-hour window of thin liquidity and widened spreads. A stop that looked safe minutes earlier can fill in that window. See Market Rollover Explained.
What this means for your account
Server-side execution records are authoritative, not the chart. Equity — which moves with the tradable price — is also what drawdown checks use, which is why a breach can occur while the chart looks fine.
If you believe an execution was wrong: send support the ticket number, symbol, exact time, your platform screenshot and what you expected. Genuine pricing faults are handled under the system-error rules — and deliberately trading a suspected fault is prohibited.