Custard usually goes like that when it's done too quickly at too high a temp. My MIL always does it when she makes custard because she always rushes it LOL. drives me mental ...
ETA - another thing when baking custard is to make your own little bain marie when cooking it - get a big backing dish, put about an inch of water in it (or enough to come half way up the side of the dish your custard is in) and then put in the oven.
Other way to stop it curdling is to add the tiniest bit of flour (probably about 1tsp for that mixture) but that is only if you are going to have to cook it faster - the slow cook is always the best way.
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