After digging around on GitHub, and working with a very patient guy from MS there, the current solution is to add this to the DbContext
protected override void OnModelCreating(ModelBuilder modelbuilder)
{
foreach (var relationship in modelbuilder.Model.GetEntityTypes().SelectMany(e => e.GetForeignKeys()))
{
relationship.DeleteBehavior = DeleteBehavior.Restrict;
}
base.OnModelCreating(modelbuilder);
}