The code below creates a TreeTableView with 10 columns numbered 1 to 10. Columns 3 and 4 are hidden.
When trying to move column 2 between columns 5 and 6, it is actually moved between 3 and 4 (can be verified in the column selection menu), so it looks like it is not moving.
When trying to move column 2 between 7 and 8 it gets inserted between 5 and 6.

So the offset that the column is moved by seems to be calculated without skipping hidden columns.

Thanks

import javafx.application.Application;
import javafx.beans.property.SimpleStringProperty;
import javafx.scene.Scene;
import javafx.scene.control.TreeItem;
import javafx.scene.control.TreeTableColumn;
import javafx.scene.control.TreeTableView;
import javafx.scene.layout.AnchorPane;
import javafx.stage.Stage;

public class A extends Application {

    @Override
    public void start(Stage stage) throws Exception {
        TreeTableView<String> table =getTreeTable();
        AnchorPane pane = new AnchorPane(table);
        Scene s = new Scene(pane);
        stage.setScene(s);
        stage.show();
    }
    private TreeTableView<String> getTreeTable() {
        TreeItem<String> root = new TreeItem<String> ("This is the root") {
            @Override public boolean isLeaf() { return false; }
        };
        root.setExpanded(true);
        root.getChildren().add(new TreeItem<String> ("a very very very long leaf") {
            @Override public boolean isLeaf() { return true; }
        });
        TreeTableView<String> table = new TreeTableView<>(root);
        table.setShowRoot(true);
        table.setTableMenuButtonVisible(true);

        for (int i = 1; i <= 10; i++) {
            TreeTableColumn<String, String> c = new TreeTableColumn<>(String.valueOf(i));
            int i_final = i;
            c.setCellValueFactory((item) -> {
                String s = item.getValue().getValue();
                return s.length() > i_final ? new SimpleStringProperty(s.substring(i_final, i_final + 1)) : null;
            });
            table.getColumns().add(c);
            if (i == 3 || i == 4) c.setVisible(false);
        }

        AnchorPane.setLeftAnchor(table, 0d);
        AnchorPane.setTopAnchor(table, 0d);
        AnchorPane.setRightAnchor(table, 0d);
        AnchorPane.setBottomAnchor(table, 0d);

        return table;
    }

    public static void main(String[] args) {
        Application.launch(A.class);
    }
}