Vegan crema that does not split when it gets warm

Splitting is almost always the acid, not the heat. Cashew cream is an emulsion held together by the nut fat and the starch you blended in with it. Add lime juice at the end, as most recipes tell you to, and you drop the pH sharply while it is cold, which destabilises it. Then heat finishes the job. Two fixes. Add the acid at the start, before you blend, so it emulsifies with the acid already in it. And add a spoonful of the soaking water back, which gives you more starch in suspension. Mine holds up to a warm filling with both of those and it did not before either.

Same principle as a cocktail emulsion, for what it is worth. Acid first, fat second, and never the reverse.