I'm programmatically creating a node, setting a sort order value to make it the first, rather than last child of its parent. Saving the node then over-writes the set order:

// ... create newNode, set some properties

// get the children of the parent, sort by sort order
// get the first (ie lowest sortOrder), subtract 1 to put the new node first
var parent = contentService.getById(id);
var sortOrder = parent.Children().OrderBy(c => c.SortOrder).First().SortOrder - 1;

// set the newNode sort order
newNode.SortOrder = sortOrder;

contentService.Save(newNode);
// newNode.SortOrder = parent.Children().OrderBy(c => c.SortOrder).Last().SortOrder + 1;

Why would this be happening?