Ann: Resignation of Non Executive Directors, page-26

If one or the other left, then less negative explanations are plausible.
But both leaving at the same time, so soon after starting, is dodgy. If nothing else, this is not good for the reputation of either women, and they would have avoided it if possible. And Naomi Scott doesn't sound like the type who can't take the pressure of working in Africa in the mining sector.
Best case scenario is what Paul suggested. i.e. that they are suddenly no longer needed and are being paid to go away.