TH3 missing labels

[couet]

Describe the bug

the attached TH3 has two missing labels. Note that changing the diction to "not optimise" makes them appear.

Expected behavior

The two missing labels should be displayed.

To Reproduce

{
  f = new TFile("th3_label.root”);
  h3->Draw();
  cout << h3->GetXaxis()->GetNbins() << endl;
  h3->GetXaxis()->GetLabels()->ls();
}

The root file is th3_label.log (renamed . log to be able to attach it).

[couet]

A reproducer. A simple macro. Is:

void testTH3Label() {

   TRandom3 r;
   int nBins = 3;
   int nEvents = 100000;
   double minRange = 1; double maxRange = 5;
   TH3D* h4 = new TH3D("h4", "h4-Title",
                       2*nBins, minRange, maxRange,
                       nBins , minRange, maxRange,
                       nBins , minRange, maxRange);

   std::vector<TString> labels = {"a1","b1","c1","a2","b2","c2"};
   for ( Int_t e = 0; e < nEvents; ++e ) {
      TString label = labels[r.Uniform(0,labels.size())];
      Double_t x = r.Uniform(0.9 * minRange, 1.1 * maxRange);
      Double_t y = r.Uniform(0.9 * minRange, 1.1 * maxRange);
      Double_t z = r.Uniform(0.9 * minRange, 1.1 * maxRange);
      h4->Fill(label, y, z, 1.0);
   }
   h4->Draw();
   /// h4->GetXaxis()->SetNdivisions(-1); // This line fixes the issue
   h4->GetXaxis()->GetLabels()->ls();
}

[couet]

Fixed by: #6391