It’s a quirk of the reporting regulation that means things like slippage/delay costs (the change in price between when I decide to trade and when the trade executes) can be negative. For example if I decide to sell at £1 but by the time the trade goes through the prices changes and it executes at £1.01 that shows up as a negative 1% cost.
Transaction cost reporting is very confusing and the numbers don’t reflect what a typical investor might expect - this is one of the better explanations:
In general for a broad market-cap weighted fund (which is naturally self-balancing) from a big provider like Blackrock / Vanguard as is the case here it’s probably not worrying about as there shouldn’t be a lot of transactions and the stocks will be highly liquid.