Clustergram uses optimal leaf-order which is causing the difference in the ordering in the results.
I have added optimal leaf order in below code. It produces same output for both the functions (above images):
rng('default')
X = rand(10,3);
tree = linkage(X,'complete','euclidean');
D = pdist(X);
leafOrder = optimalleaforder(tree,D);
label = ["a", "b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t"];
[a,b] = dendrogram(tree,'Reorder',leafOrder,'Labels',label);
cgo = clustergram(X','ColumnPdist', 'euclidean','RowPdist', 'euclidean','Linkage', 'complete','ColumnLabels',label,'ImputeFun',@knnimpute);