Fish Pie with Greek Yoghurt

If it's a high cooking temp and a low fat yoghurt , then I think it is very likely to curdle.
I don't cook many things like that in the oven but for sauces on the top I tend to start with some higher fat creme fraiche and then add in lower fat yoghurt/ creme fraiche.
When it has curdled I've rescued it by adding a small amount of slaked cornflour or arrowroot. Perhaps if you added that at the start if it would make curdling less likely.