While there is no margin requirement requested for fully hedged positions, this does not protect your orders to be closed out at one point.
Mainly this is related to spread widening. Spreads may widen depending on the product you are trading during overnight hours, over news releases, during the market opening, and during market closure, as the liquidity is thin and the volatility is high.
This can also be related to the overnight financial charges, which apply to each position if you keep them open over rollover, which occurs at 17:00 New York time each trading day.
The moment the equity on your account falls below zero, your open positions will be closed out, so it is important to ensure you have funds to support your account.
Note: Our Invest account allows 'LONG only' which means you can't "hedge" a position. Positions in our Invest account cannot get stopped out.