TTreeReader bogus data in std::vector if first element skipped

In the case of accessing a TTreeReaderArray that is from STL vector of anything (float in the example), accessing an offset other than zero can sometimes lead to garbage data if the first element has not previously been accessed.
I made a minimal example in pyROOT, as attached
The expected output should be 20 random integers, but in test case 0 or 1, the output stops making sense after the 5th entry and changes from time to time (junk memory presumably).