bug in plotPartialDependence for categorical predictors ?

Hello

I am using regression tree with two predictors : one real and one categorical (10 categories from 0 to 9). When I plot the partial dependence for the categorical predictors, the estimates are given for 20 categories instead of 10 (at 0, 0.5, 1 etc. till 9 then NaN). I know that we can not use "query point" option to change these values but I do not understand how a partial dependence can be computed for a category that does not exist (as 0.5 or 1.5). Is it a bug ? My matlab version is 2018a and my script is

T=fitrtree(MODEL(train,2:npred),MODEL(train,1),'CategoricalPredictors',2);

plotPartialDependence(T,2);

[X,Y]=extract_fig2D(gcf); % this code simply extracts graphical values from a 2D plot

>> X

X =

Columns 1 through 14

0 0.5000 1.0000 1.5000 2.0000 2.5000 3.0000 3.5000 4.0000 4.5000 5.0000 5.5000 6.0000 6.5000

Columns 15 through 20

7.0000 7.5000 8.0000 8.5000 9.0000 NaN

Thank you in advance

Vincent M.